How can we decide whether a token is JWT or not by inspecting the token?

I am in a middle of a project that uses access token identification and validation. We are trying to decide the type of the access token (basically JWT or non-JWT) after inspecting the access token. I am not sure if I can identify a JWT by identifying 3 parts separated by "." since someone can make a non-JWT with their preferred characters which may include ".". It is really thankful, if someone can help me with something.