How to determine if an animal is a familiar or a regular beast?

As a player (or DM) might use a familiar as a spy, are there ways, magical or otherwise to detect that a cat is a familiar, or just a house cat?

I’ve read the answer to Are familiars considered magical for effects like detect magic? and agree with the accepted answer that they are not. So what might be other methods for exposing the nature of the creature in front of you?

And I mean short of killing it to see if it disappears 😉