A healthy guinea pig should be inquisitive and active.
Check for the following:
Eyes: Bright and clear, without any discharge
Nose: Clean and free of discharge
Mouth: Clean. Dribbling can be a sign of problems
Breathing: Quite and regular. Should not be belabored
Coat: Dense and Shiny. Skin free of scales and sores
Body condition: Plump and well fed. No abnormal swellings