James often has very useful information on self-defense matters. The other day he linked to this very nice post on things to watch for:
A common topic in law enforcement training circles is the detection of something called "pre-assaultive threat indicators". In any police contact, officers are trained to look for physical cues that serve as "early warning signals" that a person may flee or become combative. These same cues can be useful in the civilian world as well. Being able to identify body language that indicates an imminent attack can give you the opportunity to, at best, "beat feet". Or at worst, allow you to preempt the attack with a defensive measure.
Aside from the usual gripe about the use of 'civilian', this is a very good piece. Go take a look.