In the contacts module the icon of the account type appears on the right of each contacts row. You may need to scroll across the page to see this. This could be a ClubSpark, Facebook, LTA, Google or Microsoft icon.
This indicates which account a member should be logging in with.
If you click on the icon, it will show the username of the account (or email address), which you can give to the contact if they've forgotten which method they used to register their account.