An American saying
If it looks like a duck, walks like a duck, and talks like a duck, then it's probably a duck.
(This is a popular variant; Google
doesn't bring up a standard formulation or an indication of origin, and I needed a shorter node title
Talking ducks exist only in cartoons. When using this proverb, the speaker accepts cartoons rather than real life as the frame of reference! Many a theory of mind or theory of language I have seen in philosophy class cannot adequately account for this.
Social anthropologists may be interested, too.
PS: Meanwhile I've been informed that "if it quacks like a duck" is actually 4 times as common. It's a pity Google doesn't have archives predating Donald Duck.