When someone loses a baby tooth, how would they know if the whole tooth came out?

When a baby tooth falls out, the big tooth is usually underneath the gums, sometimes you can see a little bit of the bottom of the tooth after the baby tooth comes out. Sometimes the tooth will have a hook on it, other times it comes clean out. You can tell its fully out when the gums have a small hole where the tooth use to be, if part of the tooth breaks off, the adult tooth will most likely push the tooth out once it comes in.

That's my best answer for you, you might be able to shorten it a bit if you like. Hope it helps! :)