Lot's of very normal things can tell you if someone is going to attack you, it's not mystical. Pay attention and eventually you'll start to see these things. You probably see them now, you just don't pay attention long enough to use them.
Let me add that if real violence is not in your environment, identifying clues will be very difficult even if you pay attention, for you would not know what to look for. If you are not a natural you'll need training.
Greg Steckel wrote:
As far as telling if someone is a martial artist, law enforcement, or military just by the way they move or act? absolutely can do - I have had extensive association and/or experience in all three of those environments and I can pick all of them out of crowd with just a cursory glance
I have a pair of funny anecdotes regarding military people in plain clothes thinking they were unnoticed.