Basic Trope: A woman finds a piece of hair on her husband's clothing, pillow, etc. that is obviously not her own (e.g. longer or shorter, different color, different texture, etc.) that indicates her husband is (or at least may be) cheating.
Straight: Alice is doing laundry, and finds a long, blond hair on Bob's jacket. This indicates to her that Bob is cheating on her; where else could that hair have come from?
Alice finds not only the hair, but some lipstick and a whiff of perfume. Later that day, she finds a used condom in the trash, when she and Bob haven't had sex in months (or don't use condoms).
Alice finds a lot of hair on Bob's jacket. Turns out Bob has a Catgirl (who is shedding) for a mistress.
Alice finds a wig.
Alice finds the other woman draped over Bob's shoulders.
Downplayed: Alice finds a hair on Bob's jacket that's a slightly different shade than hers, but could have come from him, maybe. She needs more evidence.
Justified: Alice needs evidence before she goes off accusing her husband of cheating on her, and an obviously foreign hair strand can be a good indicator.
Bob finds blonde hair on Alice's jacket.
Alice finds all sorts of signs that Bob is cheating (not the least of which is a strand of blond hair, see "exaggerated"), but still believes Bob is faithful to her.
Subverted: The hair (found on the sofa) belongs to their neighbor, Claire, who sat on the sofa during the Super Bowl party last night.
Double Subverted: It turns out that (after a few too many beers), Bob and Claire did it on the sofa when Alice was out of the room.
Alice carefully checks Bob's clothes for hair but doesn't find any. She concludes that Bob must be cheating on her with a bald girl.
Zig Zagged: Alice finds a long, blonde hair, and accuses Bob of cheating... But Bob isn't cheating at all, and proves it. Alice apologizes... But it turns out she knew exactly who the hair belonged to the whole time, as she's the one who's been cheating, and just accused Bob so he wouldn't suspect that Alice was sleeping with another woman.
Alice doesn't find any evidence (real or just perceived) of Bob's infidelity (whether or not Bob actually is being faithful or is just good at getting rid of/hiding evidence is another matter entirely).
Defied: Bob *is* cheating, but carefully destroys or gets rid of any evidence; he uses a lint roller to remove any foreign hairs from his clothes, he lets his mistress dispose of the condoms they used (or simply doesn't use them), covers up his mistress' perfume with copious amounts of his own cologne, uses one of those detergent markers to get rid of lipstick stains (or does his own laundry), deletes his emails and text messages, and acts super sweet to Alice so she won't suspect anything.
Discussed: "Bob! Whose hair is this on your jacket?" "This isn't a chick flick, Alice. Do you know how many people I bump into on the subway every day?"