Is it possible to discern if a username/email is already taken when using Signup endpoint via Axios? (React)

I have successfully been able to create users using a custom form and the Signup endpoint via Axios when working on my React project recently.

My question is this: Is there any good way to see if a username is taken before a signup occurs, or consequently…is there a way to discern that specific problem via the result from the Axios request?

I believe (from what I have tried) you get a 400 “bad_request” error when this is attempted.

Do I just need to assume that all of these types of errors are for that reason? Or would I need to check (somehow) for the existence of said user BEFORE attempting to create the user in order to handle this specific error?