1. If you ever find yourself outside her window without her knowledge.
I know a flashlight is normal to keep in the back of your car but military grade night-vision goggles? Dude, pick up the empty red bull cans, the trail mix bag and whatever you were using as a toilet and get out of her bushes. Go home and watch Big Brother instead. Despite what you may think, you can become quite emotionally attached to those people as well. And they ignore you like your ex is doing while she sits there eating dinner with her new boyfriend.
2. When you go to type facebook in your address bar and their profile is suggested before facebook.com.
In the new age of Facebook and Twitter (permitted stalking), you don't even need to leave your house to become a creeper. When you make someone's profile page your home screen, you have gone a step too far, especially after she removed you as a friend.
3. When you are at her work more than yours.
Dude, there is no reason for you to be at Starbucks 8x a shift every day she is working. Unless you are a script writer (or blogger), there is no reason for you to stay in any coffee house for hours at a time (and no, "free wifi" is not a good enough excuse).
4. Shrine of any kind.
Any kind of memorial or shrine, especially one that has a statue or totem. Granted the best example of this is Helga from Hey, Arnold! but it doesn't need to be that elaborate (or creepy) to be stalkery. Is "stalkery" a real word? It is now. The closes to a shrine you can have is a photo album that doesn't get opened. An open photo album on a coffee table opened up to an ex is just as bad as a statue of them with the medium of choice being their chewed up gum.
5. Going through her garbage.
The only time this is acceptable is when you are in a relationship and she loses something valuable. But when you just got done crying about how you miss her and decide to go dumpster diving outside her apartment at 2 am like a homeless guy looking for a midnight snack, you have crossed a line or two.