Listen for quite a while. If the answer to any of these questions is yes, leave the store. A good pet shop should have all healthy rats. The eyes on the rat should be clear, not runny or cloudy. If a ray wheezes or sneezes, it is a sign of a respiratory infection.
Respiratory infections contribute to the deaths of many rats.
The litter in the cage is a very important factor in the health of a rat. Overcrowding is another problem that contributes to sickness in rats. If the cage looks dirty and smells bad, it probably hasn't been cleaned in a while. It may be harvesting bacteria, lice and disease. All rats are sweet.