How can you tell if a dog is spayed?

Ferrets that are adopted from pet shelters, or bought from pet stores will be neutered. The largest breeder of ferrets in the US - Marshall Farms will have two blue tattoo dots on the ears indicating it has been neutered. Private breeders usually will not sell a ferret without the agreement that you have the ferret neutered

If the above statement does not answer your question, refer to related question below on breeding ferrets - if a ferret is not spayed and comes in season, you will be able to tell by physical signs that change on a female ferret.