1. Observe any “unnatural” habits.
Obsessing too much over odd topics. If she (or he) obsessively talks about Twilight or seems to only want to talk about fuzzy bunnies 24/7 you may be flirting dangerously close to crazy town. Or if he/she wants to talk about changing you (changing your hair, clothing…personality) to the point where it’s all that the other person wants to talk about you may want to reconsider the relationship.
Extreme mood swings. Are you unsure which guy or girl you are going to get from moment to moment? Do you find that you could be having a completely rational, lovely conversation and then suddenly, without warning he/she gets upset or angry? Or does he/she become insane and completely irrational after having a few drinks, but then deny or blame others the next day for his/her behavior?
Scary bouts of anger or violence. Does he/she seem to enjoy hitting you or is into S&M (even after you tell the other person you like to kick intimacy old school)? Some people may also find auto-asphyxiation to be arousing, which may not necessarily mean that they are nuts, but if you are uncomfortable it may not be the relationship for you.
Odd habits. From odd collectibles to an insistence on making you wear a special jumpsuit upon entering his/her home; identify any outlandish habits that make you feel uncomfortable or seem off base.
2. Check out his/her circle of friends/family members.
Are his/her friends a little off? You can tell a lot about someone by the friends he/she chooses. Consider what the circle of friends enjoys doing or how they behave in public.
Does he/she seem to only have casual acquaintances? Often crazy people have a hard time maintaining friendships for a number of years. Does your date have long time friends or does he/she seem to only have a handful of folks typically called upon for nights when they go clubbing (this doesn't include someone who is new in town and only starting to meet people)?
Have you met his/her family or have you been told they “live abroad” or were “killed in a freakish accident?” While some unfortunate people have been orphaned or experienced tragedy in their life, you certainly want to be cautious if you believe you may have a Ted Bundy type on your hands.
3. Examine your interpersonal relationship with him/her.
Does he/she seem to be obsessed with you? Consider how your date regards you. After only dating a few days has he/she started to call you a boyfriend or girlfriend? Has he/she talked about marriage or a long term commitment during the initial phase of dating? Flying into something serious should send up the red flag.
Do you have difficulty communicating on a regular basis? Does he/she never really understand what you are talking about or misinterpret what you say (to your detriment)?
Does he/she have problems with other people and exhibit inappropriate behavior? Do other people tell you that he/she is nuts or have you observed odd behavior between your date and other folks?
4. Identify your relationship goals.
You only want to have fun. If you are only interested in a short term relationship then you don’t have to worry about flooding your gene pool with a crazy. However be especially cautious about how you eventually break it off with the other person--you don't want a casual relationship to turn into stalker situation.
You’d like to find “the one.” If you suspect the person you are dating is psychotic or even a little nuts, consider ending it sooner rather than later. The more time you invest in this relationship, the less time you have to find the person who could possible be as serious lifetime partner. Plus the other person will become more invested and could make the break up even more difficult.
You aren’t sure what you want from the relationship. Still on the fence about whether the other person is nuts? If you have time to kill and want to see where this thing is going stay the course, however veer off if you have a strong feeling that crazy is at your house.