What to look for... well I would make sure to check to make sure the animal has no small black specks on it (look around the eyes and heat pits - also check out the caging as well) to make sure it does not have mites or ticks. You want to listen to make sure the animal is not wheezing or raspy. Check the vent to make sure there is no discharge or prolapses. The heat pits should be clear, and the inside of the mouth should be a nice healthy pink.

Those are few things off the top of my head. I hope that helps a bit.