I was never a religious person so many of the points mentioned in the posts don't apply to me. I never prayed or said "Alhamdulilah" and stuff. Still, here's my list.
You know you're an ex-Muslim when...
You might also get the urge to put something irreverent, like 'place bacon upon him' (Muhammad)
... the above quote makes you chuckle.
... you can buy food from the non-Halal-certified stores (for me, during lunch at work, despite my Muslim friend's disapproval

) without feeling guilty.
... your Muslim/God-believing friends are talking about how it's easier to prove the existence of God than argue against it, and you're sitting there not saying anything and feeling like you're about to burst because you're still a closet apostate.

... you don't respond to "Assalamualaikum", or you do so very hesitantly.
... you eat pork, club and drink without feeling guilty.
... your period very conveniently comes everytime someone asks if you want to pray.
... you see people posting on Facebook/Twitter about how Allah is great... and you feel sorry for them and how brainwashed they are.
... you scoff when you hear Muslims talking about how wearing a hijab is a position of honour and that Muslimahs really do have many rights in Islam.
... you feel ridiculously stupid thinking about the times when you actually used to defend Islam online and proclaim yourself to be a proud Muslim. *facepalm*
... when people ask, "You're a Muslim, right?", you awkwardly answer "Erm, well that's the religion I was born into..."
(lastly, applicable to atheists in general)... you laughed when Ricky Gervais closed the Golden Globes with "And thank you to God, for making me an atheist."