Are there anyway that we could used to check that the credential existed but not create new account?

Since we could get and link credential with many sources. Google, Facebook, GooglePlay, GameCenter, github, etc. I need to have some way to verified that, is the credential already existed and linked with some ID or not

But I don't want to signed in with that credential immediately. Because I need to generate custom userID. If it linked I will let it signed it. But if it not I would like to have user pass the initial process first and the server will minting the token to let user signing in and link their credential with it after that

Is there any API to do that, especially in client?

If not I would like to request API Auth.getUserIDFromCredential(credential) or Auth.signInWithExistingCredential(credential) that will signed in only if the credential has already been linked with any account