How to check if the USER is already created in the database or not in SQL?

Is there is a way that from it I can know if the user(not the login) is already created in the database? I mean the user not the login, since, I know how to check for the login. I need to check for the user that is created inside a specific DB & a role assigned to it.

This is the code for checking for the login:

SELECT name FROM sys.server_principals WHERE name = 'test_user'

but how about the user? Since I need to create the user and assign a role to it if its not created. Otherwise, I will continue without creating.

Thanks