How to know if current user is an linked account?

You could use

\Drupal::service('externalauth.authmap')->getAll(\Drupal::currentUser()->id());

This will return an empty array if the current user is not linked with an external service, or an array keyed by provider ID if they are.

A more developer friendly service method could be added for this purpose I guess..

Thanks @Sven

Automatically closed - issue fixed for 2 weeks with no activity.