When he mocks you in front of his friends about what he used to tease you for only in private.
When he starts to make comments about things that used to be of no consequence--e.g. wearing jeans too often or having messy hair in the morning--more than he compliments you.
When he lies about his whereabouts... even to say he wasn't going somewhere just to avoid inviting you. Especially when you couldn't have cared less about going along in the first place.
When you realize that the past two years didn't mean as much to him as that last 20 minute one-sided sex session.
When he stops contributing to your happiness and starts to hinder it.
When he text messages you from another car 2 hours into an 8 hour road trip home with his friends that you're "incompatible" and you "need to say bye and move on." Or when he threatens you with a restraining order after you request that he stand up and be a man and breakup with you in person.
Sorry, I'm a little bitter right now.