Wheezing is more apparent after exercise, so have the rat run around for a while. Turn the rat over and gently feel his belly. Check the rats nose, eyes, and ears. Feel the rat as you pick it up. It should feel hefty, not frail or thin.
Sick rats tend to hunch up and be unresponsive.
Although fear can cause a rat to tense up, in time it should relax. By the way, diarrhea can also be caused by fear. This is a sign of sickness. This is a picture of male versus female babies. Older rats are easier to check, because the testicles are very prominent.