I surveyed a few friends at Touch Supper Club last night about this topic. So I can't take credit for all of 'em.
10. If the person cramps your style.
9. If they mention their: wife, husband, boyfriend, girlfriend, "baby's momma," or "baby's daddy."
8. If they talk about marriage in the sentence before asking you out on a date.
7. If their email address or screenname contains any vulgarities or references to genital or breast size. And it's likely not true anyhow. Duh.
6. If they smell.
5. If you meet them in line at the STD testing clinic.
4. If their choice of jewelry is a psychiatric hospital wristband.
3. If he or she sleeps in their car, under a bridge, at a shelter, or in a refridgerator box.
2. If they happen to mention that their ex is still "missing," and you notice blood on their clothes.
1. If you recently saw them on "To Catch a Predator," "America's Most Wanted," or "Cops."