When you're trying to crack a big conspiracy, sometimes it can be hard to get people to open up to you because they're more scared of the conspirators than they are of you. At other times the conspirators are well-connected and have judicial protection for their secrets. But everything needs to be paid for somehow, and tracing the money can often get you the information you need.
Such Forensic Accounting is often viewed as boring and/or headache-inducing by the characters, because it involves a lot of staring at numbers. Despite the name, this can easily be applied to paperwork such as cargo manifests. Either way, there is usually a forensic accountant who hits the high points about his work in his report to the detectives, who are simply pleased that they have the evidence to bust a crook from a purely intellectual resource.
Quite common in Crime and Punishment Series of all types. As such, don't bother listing every example from such series; one or two per show will suffice.
See also Intimidating Revenue Service, where this trope is a common tool. Not related to Follow the Money, even though that's a stock phrase often used to describe this trope.
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While the precise procedure it's not shown on-screen, this is how Ra's Al Ghul was able to find Batman's real identity. As he explains, a thorough examination of Bruce Wayne's expenses revealed some in-congruencies, which were the purchases of the Bat-gear.
In the film adaptation of V for Vendetta, Detective Inspector Finch decides to go through government tax records while trying to find out details about the terrorist V. This thread of investigation eventually leads to him uncovering sensitive details about the conspiracy the Norsefire party used to bring itself to power. (In the original comic Finch got the information from the FATE supercomputer, which wasn't in the movie.)
Finch: One thing is true of all governments; the most accurate records are the tax records.
Batman Begins has Rachel's boss stumble onto the League of Shadows' plan after he discovered a cargo ship had arrived in Gotham's port with one more cargo container than its manifest said it embarked with. Unfortunately a couple of Dirty Cops off him before he is able to tell anyone.
A subplot in The Dark Knight. Bruce Wayne examines Lau Enterprise's financial records to confirm his suspicions that the company is a front for money laundering. Later, one of Wayne's own accountants figures out that Bruce Wayne is Batman, just by taking a closer look at Wayne's records and noticing how much money is going towards research & development projects that never get released. Nothing comes of the latter due to Lucius Fox giving the accountant a reality check.note To wit, that blackmailing a guy who spends his nights beating the crap out of criminals isn't exactly the best idea.
In The Other Guys, Will Ferrell's character is trained as a forensic accountant, which initially makes him a joke to the other cops, but eventually proves to be just what was needed.
The film All The Presidents Men made the phrase "Follow the money" a part of the political lexicon and popular culture. In the film, it is whispered to reporter Bob Woodward by Deep Throat as a way to cut through the lies and deceptions and find the truth about the Watergate scandal. This is a fictionalized line created by the movie, but nevertheless catches the spirit of the process perfectly.
In Komarr, Miles has definite evidence that a plot exists, but no idea what the plot is about, so he calls in some ImpSec analysts to see if they can reverse-engineer the plan from the purchase orders.
In The Borders of Infinity Miles has his mercenary accountants infiltrate the enemy prison camp offices in order to facilitate the major operation he has planned there.
In Anansi Boys, Charlie Nancy's employer Graham Coates tries to frame Charlie for his own dirty dealings. Charlie is arrested, but released without ever going to trial, because the police are able to correctly deduce from the agency's financial records that Charlie had been framed, and couldn't be responsible.
Discussedsubverted in a Lord Darcy mystery. Lord Darcy observes that while trying to solve a murder occurring on a train, another detective was led astray because his experience is in this, where conspiracies are easy to form and hard to figure out.
Live Action TV
One episode of Castle had the team trying to find out who a slain plastic surgeon's last mystery patient was. Castle's solution was to get a warrant for the hospital's billing records to see who paid for the thing.
Whether it yields a lead or not, most episodes will at least mention checking someone's financials.
Stargate SG-1: "Politics" had Senator Kinsey get read into the stargate project after, in his capacity as chairman of the Senate Appropriations Committee, he demanded to know what the heck the $7.5 billion item in the Air Force budget labeled "Area 52" was.
One slightly weird example comes from a Law & Order episode with detectives Briscoe and Logan where a woman shot a man in an alley. The woman claimed self-defense, because the victim tried to rape her. Briscoe and Logan were gathering facts about the defendant, and discovered that she lived in a ritzy apartment beyond what a mere secretary could afford. Her rent checks came from the construction office of a reputed mobster. Cue Plot Twist.
The Good Wife: "Waiting for the Knock" has one of Lockhart/Gardner's major clients, drug dealer Lemond Bishop, come under threat of arrest. Their bankruptcy court-appointed trustee, an accountant by training, is brought in to help them figure out what's going on.
Data: Captain, I have been considering the problem of the missing ship. Although there is no record of a launch to the Ficus sector, which would not be unusual considering the chaos of the early twenty second century, someone had to load that ship.
Picard: The manifest.
Data: Yes, sir.
Picard: There it is. SS Mariposa, loaded 27th November, 2123.
In the gimmick TV series Push Nevada the protagonist is an IRS accountant who found discrepancies in the books of a casino.
Used frequently in NCIS: Los Angeles (typically Eric's and Nell's job), though one episode put a twist on it: LAPD was only too happy to hand over an investigation to them because the suspect was old-school, using all-paper records instead of easily searchable computers. Boxes and boxes of them.
In The Wire tracing dirty money is one of the specialties of detective Lester Freamon, who also instructs his fellow cops in the art. This police work usually meets a stern opposition from the higher-ups, since drug money funds political campaigns, but the few times he can use it, he compares it to a Boom, Headshot.
Lester: You follow drugs and you find drug addicts and drug dealers, you follow the money and you don't know where the fuck it's gonna take you.
On Suits this is one of Louis Litt's specialties. He can work his way through mounds of legal paperwork and figure out exactly who owns what and who is paying whom. In a flashback he easily figures out an embezzling scam from looking at some financial documents over lunch and even correctly points out that someone is being set up as a Fall Guy. This turns into a subversion when he also fails to realize that he is the fall guy in question.
On Necessary Roughness Dani and Nico are trying to figure out what exactly is going on at V3 and find out that the company donates money to a charity in the Cayman Islands but does not claim it on official financial documents. The charity is a front for a slush fund that V3 uses for bribes and payoffs. The money is further laundered through a small auto body shop in New York. Examining the shops accounting records finally reveals who is getting paid off. Dani and Nico now have to figure out why all those people are paid such large sums of money.
More than half the cases on White Collar involve taking a look at the books somewhere in the episode. It's right there in the name. Peter was hired by the FBI on the strength of his accounting skills, which is very much Truth in Television.
Plays a part in Person of Interest; Finch following the money usually gives Team Machine clues to the POI's troubles or helps build a background profile that Reese can use.
Call of Cthulhu has an Accounting skill, so the writers of official supplements sometimes threw in a clue that required that skill to find.
Shadows of Yog-Sothoth adventure "Devil's Canyon". While examining the papers in von Varnstein's office, if one of the PCs makes an Accounting roll he can discover an order for special camera lenses, a clue that tells the PCs that the camera lenses they discover later are important.
The Fungi from Yuggoth
Adventure "The Dreamer". While searching Herbert Whitefield's office the PCs can find bills and receipts. If one of them makes an Accounting roll he can determine that Whitefield is deeply in debt and late on all his payments - a clue that gives him a motive in the disappearance of his client Paul LeMond.
Adventure "Mountains of the Moon". If the PCs break into the NWI mining office's administration building and Johnathan Harris' office they can find the site's business records. A successful Accounting roll will discover that even though the operation is performing at peak efficiency, it's still losing a phenomenal amount of money. This is an important clue that the purpose of the site is not to make money and that there's something unusual going on.
Cthulhu Now adventure "The Killer Out Of Space". If a PC makes an Accounting roll while examining the books (accounting records) at Buddy's Best Wrex he realizes that they aren't correct. The books are actually false: Buddy keeps the actual books at home.
Dreamlands adventure "Pickman's Student". While going through Blakely's papers a PC can make an Accounting Roll. If he succeeds he finds receipts for four of Blakely's paintings, with the addresses of the people who bought them. Since the PCs must find the paintings in order to succeed this is a vitally important clue.
Horror on the Orient Express. Successful Accounting rolls are useful twice: while examining Makryat's account books they reveal that he bought and later sold a special train set, and while studying the Gremanchi Doll Work's records they show how the Conte ordered the purchase of the Left Leg and later used it.
The Asylum and Other Tales adventure "The Asylum". In Dr. Freygan's bedroom there's a wall safe behind a Concealing Canvas. It holds an account book listing the asylum's income and expenses. If a PC makes an Accounting roll he will realize that there's no way the asylum's income could cover its expenses. This is a clue that Dr. Freygan is making money another way: by helping bootleggers smuggle alcohol.
Masks of Nyarlathotep chapter 4 "Kenya". If the PCs make an Accounting roll while reading Ahja Singh's ledger, they discover listings of shipments to addresses in Hong Kong, London and Egypt. These are important clues that will allow them to investigate a world wide (and world-threatening) Cthulhu Mythos conspiracy.
RolemasterShadow World setting, supplement Kingdom of the Desert Jewel. Kohan Traska, the Advisor on Internal Affairs, has a room in the Royal Palace in the Gethryn capital city of Ketaum. Among his papers is a list of the revenue from trade items sent to the capital from the nome (province) of Shii-Magna. If the PCs analyze it, they can discover that the totals have been skewed in the nome's favor. This is because Traska was born in Shii-Magna and is a friend of its Karsha (governor). The GreatKing (ruler) of Gethrya would be interested in this (to say the least).
Chill had an Accounting skill available to characters. The text specifically said that it could be used to examine financial records to determine if embezzlement, money laundering or other kinds of financial crimes had occurred.
In the adventure "A Thousand Words", Powder used a layered series of withdrawals to remove the funds from his bank account to pay for the warehouse. If the PCs can read the books and do some computer hacking, they can find out what he did, which will give them his current location.
In the adventure under "Government Purity Goes Down", the PCs must investigate the disappearance of Jessop Hills' government education funds by examining the books of the City of Los Angeles school board' and finding out who's siphoning off the money.
In Alpha Protocol's backstory, Alan Parker went undercover as an accountant. His words to Mike Thorton on the subject provide the page quote.
Forensic accounting is a common tool used by law enforcement agencies in real life, particularly in regards to organized and white-collar crime. The Other Wiki has additional information here.
Al Capone was finally sent to prison for tax evasion rather than booze smuggling or murder. Nobody was willing to testify against him for his violent crimes, but, it was fairly obvious his lavish high-profile lifestyle was well in excess of what his declared income could ever support.