Perhaps the leading expert on the subject of facial expression is Professor Paul Ekman, whose course I had the good fortune of taking some years ago. I still have one of his books, so I went and looked up the specific details, they are as follows:

Anger will show itself in the brow, eyes, and mouth parts of the face. The eyebrows will lower and be drawn in together. The eyelids will tense, and the eyes will appear to stare in a hard fashion. The lips will be tightly pressed together (most often) or else parted in a square shape (such as when the teeth are gritted and a person is a state of rage about to attack).

I hope that helps.

Rob