Book 2: Mr. Chapman is severely embarrassed when an "Andalite Bandit" sneaks right into his basement as the family cat!
I am assuming that like all good cat owners, Chapman would have had his male cat spayed. However, if an Animorph morphed the cat, the cat morph would have all his private parts. Therefore, to make sure that his cat is not an Andalite, all Chapman need to is look to see that the cat is spayed. If it isn't, it is probably an Andalite fake.