A hamster might be a small animal, but choosing a healthy pet is important, no matter what the size. Before bringing a hamster home, examine the animal's body and get to know its personality to ensure it's the right pet for you. Look at the hamster's surroundings at the pet store, too; the cage should be clean, offer easy access to fresh food and water, and not be overcrowded with other animals. Hamsters kept under good conditions will be less stressed and have less exposure to disease, meaning your pet will be likelier to live longer.
Examine the Body
Take an opportunity to pick up the hamster and handle it for a few minutes. Look at the overall body condition: a hamster should be neither fat nor skinny, with no swellings. The hamster's coat should be well groomed—that is, fluffy and smooth—with no bare patches where fur has been pulled out. Check for soiling around the rear end or wetness on the tail, as this may indicate a problem with diarrhea.
The hamster's eyes, nose, and ears should be clean and free from discharge. Check the fur around the eyes and nose for signs of wetness, staining or crusts. Although it might not be possible, try to get a look at the teeth, as they should not be overgrown and should be well-aligned. Also, check for wet or matted fur on the chin.
Observe the hamster's breathing, which should be quiet and not labored, with no wheezing, clicking, or gurgling noises. Now, put the hamster down and watch it move around. It shouldn't display any signs of lameness, stiffness, or reluctance to move. After you've handled the hamster, observe its breathing and attitude and whether it seems out of breath or lethargic.
Take a look at the feces in the cage, as well, to be sure it is solid and not runny.
Check out the Personality
Hamsters should be bright and curious in attitude and never lethargic. This can be a bit hard to judge in a nocturnal animal like a hamster, but with attention—and perhaps some very gentle prodding—a hamster should wake up and seem to be inquisitive about what is happening.
Observe how the hamster reacts to people. Although most will be skittish at first, try to pick a hamster that is relatively calm about being approached, as well as OK with being handled by people.
Disorders and Diseases of Hamsters. Merck Veterinary Manual.
Owning a Pet Hamster. VCA Hospitals.