This is a clue as to what the conditions are that the rats are facing. When you see the cages, first look to see if they're clean. Then look and see what kind of litter is used. If it is, leave. See what kind of food they're using.
Rodent blocks are good.
Look at the atmosphere. The answer should be yes. The answer should be no. If the answer is yes, ask if the rats have been treated. But if you're not, move on to the next store or breeder on your list.