Janie, Your face.

Pieces have moods and story lines. Your intuition can be your guide. Or maybe you can read up on the ballet. Or better yet, ask the director.  

It would be very respectful to watch the performing dancer, unless there is something else going on in the piece. 

Things I have witnessed are: being disengaged, yawning, frowns, HUGe smiles when someone comes on stage, eyes down cast, laughing with neighbor, shaking feet. AND connectedness, engagement, expression changes that integrated with what was happening, eye contact with dancer and /or audience, excitement, beautiful form and position.

If you do not have a resource to ask, get a discussion going among your peers.

Watch other performances. You'll connect with what feels in sync. Just notice with out judgement or comment.