Check all users accounts in DB

Is there a way to return all addresses of the users in the DB along with their account links. Im trying to see if will work before giving the option to the user. I can see if the account is already linked to the current user, but how would I check if it is linked to any other user as well?

You can do that in a cloud function with master key. With master key you can get the info for any user.

You pass the master key through the request ? Or you just enable it true ?

It is enabled by default in cloud code, you only have to use the specific syntax with master key parameter