Not everyone is able to buy from a breeder so it’s always important to try and buy from reputable pet shops. Check the cages to see if they are clean and the animals look well cared for before purchasing, it can save a lot of heartache later on.
A healthy hamster should look clean and well groomed, no discharge from the nose or bottom, clear bright eyes and alert ears (unless he/she has just woken up in which case they will be folded slightly). They should be plump to the touch but not too fat or too skinny. Watch them for a while before selecting one to see if they look alert and curious.
This picture shows a hamster in the prime of health, if you aim for a little one who looks like this you’ll hopefully have years of enjoyment from your pet.