When selecting a rat from a pet store or animal shelter, make sure that all the rats look healthy and alert, and that the animals are kept in clean and comfortable conditions. If any of the rats look sick, or if there is evidence of diarrhea or any bad smell in the cage, the rats may have been exposed to an infectious disease. Individual rats should have clean, shiny fur, and the eyes and nose should be clear of any discharge.
Rats that have been handled should not be too skittish and are often naturally curious about humans. Because they are nocturnal, it is a good idea to view rats in the early evening when they are more likely to be fully awake and alert.