Assuming you would like tips on how to tell if a rat is a baby, there are several things you can look for. First, check the size of the rat. A baby rat will be much smaller than an adult rat. Second, look at the fur. Baby rats have softer, fluffier fur than adults. Third, observe the behavior. Baby rats are more playful and active than adults. Finally, listen to the sound of their voices. Baby rats have higher-pitched voices than adults.