I just read an interesting article from BlueSheepdog.com. They are a site dedicated to law enforcement officer training and safety. The article I read was by Lt. Jim Glennon and discussed the indicators or tells that someone is about to attack. While it is written with the law enforcement community in mind, I think much of it applies to the rest of us.
Glennon identified a number of pre-attack signs including targeted glances, clenching of hands or teeth, and eye blinks.
The whole article can be found here. Of course, no amount of knowledge about indicators is worth anything if you are oblivious to what is going on around you.