1. You don't fit in.
2. Your boss doesn't like you and you don't like him or her. If your boss never asks your opinion, and never wants to chat or have lunch with you, and if you disagree with her agenda and dislike her style, your days are numbered.
3. Your peers don't like you. Feeling isolated, gossiped about, and excluded from the inner workings of the organization is a very bad sign, as is feeling that you're not part of the team and wouldn't socialize with your colleagues even if they asked you.
4. You don't get assignments that demonstrate the full range of your abilities. "Watching all the good assignments go to others, while you're given the ones that play to your weaknesses or are beneath your professional level, should tell you something.
5. You always get called upon to do the "grunt work".
6. You are excluded from meetings your peers are invited to. Sound familiar? If it's painfully clear that your ideas aren't valued, why stick around?
7. Everyone on your level has an office. You have a cubicle in the hallway.
8. You dread going to work and feel like you're developing an ulcer. Ah, here's yet another of your symptoms, and a particularly nasty one at that.