When Discourse is used as the SSO provider, you can check if a WordPress user has a Discourse account by checking if the value returned by this statement is empty:
get_user_meta( $user_id, 'discourse_sso_user_id', true );
You’ll need to do something to handle the case where the user doesn’t yet have a Discourse account. Creating the user via the API and adding them to the appropriate groups is probably the right solution. It will take some work to set that up.