Where to begin? The show's entire premise centers around building trust between the relationship of a guy (who has a Photographic Memory) and his boss, who knows his deep, dark secret. The Ho Yay pretty much writes itself.
In the very first episode, Harvey decides to risk his job in order to give a chance at another life to a guy he'd just met. It's mostly because of Mike's Genius Bonus, but still.
Hell, Mike and Harvey meet through a Rescue Introduction, which is normally used when love interests are meeting each other for the first time.
The pilot alone is chalk full of moments, from Harvey saying "Super hot" when asked how his new associate is doing, to threatening to kick Louis' ass if ever he tries to fire Mike again.
Come to think of it, Harvey is very possessive of Mike...
And then there's what can only be described as a smitten look◊ on Harvey's face when Mike shows up, talking about how he got Joanna to agree to testifying.
One of the topics Harvey and Mike clash most frequently about is Mike's friend Trevor. Harvey keeps telling Mike to get rid of him. Jealous, much?
By 1x02, everyone knows Mike as Harvey's "golden boy". And even Louis tells Mike, "No wonder Harvey's so crazy about you."
In 1x03, Mike gets Harvey to fist-bump him. And right before Harvey relents, he smiles very briefly. No one else gets that kind of look from Harvey Specter.
Mike: You know you want it.
Throughout most of season 1, Mike frequently accuses Harvey of supposedly not caring about him. Harvey does express the sentiment a couple of times, but come season 2, he and Mike have gotten so close that he threatens to quit the firm if Mike is fired. Woah.
Harvey has a lot of pet names for Mike, some of them being "hotshot", "lippy", "Juliet" and "puppy".
Not to mention, Louis frequently compares Mike to a pony.
Harvey and Mike like to give each other long, piercing stares at least once an episode.
They also manhandle◊ each other so much you sometimes forget that Mike is dealing with his boss.
Harvey and Louis have a rivalry and although it's not exactly Foe Yay, they have fought over Mike on occasion.
No need for Foe Yay there when Harvey has Travis Tanner...
Harvey: Go easy on him.
Harvey: Good boy.
Dialogue from 2x04:
Louis: (about Mike) What can I do to him?
Harvey: What do you wanna do to him?
Mike knows that Louis' parts are tingling.
And Harvey didn't wanna know that or know how Mike knew that.
From the same episode:
Harvey: (after Louis and Mike work on a case together) You guys thinking about moving in together?
Way back in 1x02, Harvey and Louis had a similar exchange when Louis forced Mike to work on a case for him. Harvey is really transparent...
In 2x03, Mike breaks up with Rachel because he decides he can only be in a relationship with someone he can share his secrets with. Who does that leave? Gee, Mike, you might as well date the one responsible for you having that secret to begin with.
Not to mention the fact that Mike did it all because Harvey told him to, which shows—no matter what—Harvey comes before everyone else in Mike's mind.
Mike hero-worships Harvey so much that he's rendered flustered and speechless when Harvey tells him he's proud of him in the second season premiere.
2x03: Louis replays Harvey's voice again... And again... And again....
In 2x01 Harvey can't fire Mike, despite being ordered to. He's all set to do it, and then Mike shows up and he just can't do it.
2x02 has the infamous pineapple scene. Complete with an "I love you" and mutual food-sharing that neither of them bat an eyelash at. (Note that later in the episode, Mike is seen still carrying around the pineapple slice.) Guess the writers were feeling generous that day...
Mike could totally pull that off. He has soft features.
Harvey also answers without hesitation (when Mike asks if he would be Spock or Kirk) that Mike would be Uhura.
In 2x05 Mike goes and tells Harvey about Donna, because he hero-worships the guy enough to believe he can fix it, even after finding out that Donna destroyed evidence.
The reason Harvey votes to settle in 2x07 is pretty clearly stated to be because Mike told him to.
Mike didn't just tell him to, he essentially threatened to leave Harvey if he didn't.
Donna and Rachel's first meeting in the flashback in 2x08 would not be out of place in a romantic comedy.
Donna: I'm Donna, I know everything.
In 2x10, Harvey is at the lowest he's ever been on the show: Jessica lost her position as managing partner and Hardman successfully took over the firm, Harvey was reassigned to a lower office and has to deal with menial, tedious assignments by a vindictive Louis, and top it all off Louis forbids Mike from helping Harvey with any of his cases. How does he deal with it? He goes over to Mike's place to get stoned and comfort Mike about the loss of grandmother, sharing a good deal about his personal life in the process!
In the season 2 finale, when Jessica blackmails Mike into betraying Harvey. Poor guy just looks so heartbroken through the whole thing. Particularly when Harvey finds out about it. He also lists Harvey among the people and things he's lost when Rachel confronts him about not signing a letter she intended to send to Harvard challenging her rejection.
More specifically, he says he's lost "everyone I love".
Harvey: (to Jessica during a party) Don't mind me, my date just got here. (The camera cuts to Mike.)
The phrase "Do you trust me?" pops up in conversations between Harvey and Mike every couple of episodes.
Season 3 is already out-gaying 1 and 2 combined by far. The first ep has Harvey and Mike fighting, with Mike even dreaming about Harvey forgiving him. Harvey, however, feels Mike has betrayed him and spurns Mike's advances, giving him endless speeches about not being loyal. He acts like he's been cheated on, and Mike even begs to be taken back.
3x02 is full of moments for pretty much the entire cast and is purposely played out like one big romantic drama:
Cameron Dennis snaps at Jessica and admits that what bothers him most is that Harvey picked her over him.
Harvey and Mike have an intense private conversation in Harvey's office wherein Mike begs to be taken back for the nth time. Harvey sidles up to him and says very firmly: "We're done". It's totally a break-up scene.
Rachel spends the episode fighting with Mike and Donna because she feels they both keep picking Harvey over her. Harvey/Mike and Rachel/Donna shippers were made very happy.
Harvey: (bitterly, to Mike about Rachel) Congratulations. I hope you two are happy together.
Even Louis knows:
Louis: I need the 411 on the break-up.
Donna: Are we in high school?
Louis: Life is high school. And I need to know if the prom king and his boy wonder are still an item.
Donna: What high school did you go to?
Louis: Donna, it's a simple question. Did Mike and Harvey break up break up? Or did they take a time out, and like say to each other, "hey, let's see other people to make sure that we're sure?"
The whole episode's plot deliberately parallels love triangle movies with Louis and Harvey fighting over Mike. Louis asks for Harvey's "blessing" just to be able to date, err— take Mike as an associate, like a friend asking if he can date a friend's ex. He even specifies that Harvey can't swoop in, object and take Mike away when Mike eventually will say yes. Harvey brushes it off at first, which destroys Mike, who allows himself to be wooed by Louis. He starts enjoying actually spending time with Louis and says yes to being his associate... And then Harvey decides then and there to walk in and protest, asking Mike not to go through with it. The final scene deliberately echoes cheesy climaxes wherein the main character shows up at their true love's wedding. Especially the dialogue:
Harvey: (appears in the doorway behind Mike) I can't let you do this.
Mike: Let me?
Harvey: Don't do this.
Mike: (sadly) I already said yes. I can't go back on my word.
Harvey: You're not going back on your word... You're going back where you belong.
Seasons 3 and 4 take the Ho Yay between Harvey and Mike even further.