swift - Checking if a user already signed up -

i built custom authentication system using firebaseauthentication tokens.

my signup / login flow should work this:

1. user presses login button
2. my server generates authentication token , sends client
3. check if user exists (in 'auth' table or in database?)
   • if true: sign in using firauth.auth()?.signin(withcustomtoken:...
   • if false: show form to enter custom information (name, etc..)
     • sign using firauth.auth()?.signin(withcustomtoken:...
     • save custom information database

my question is: how can find out if user has signed up? publicly accessible database uid's way go?

this opinion based, yes, use standalone db stores each user's username has signed up. required quick web request through php file querying rows returned username.