How to check which users have been created by specific stored procedure in sql server?
You can't without storing something uniquely identifying, particularly if (when) users/logins are dropped and recreated by other processes than the stored procedure.

Perhaps you could store just the principal_id and create_date from sys.server_principals, and the same from sys.database_principals; nothing identifying in particular, but the pair together should be a good indication that these are the right rows.