First click on "my account" then "earnings" then "referral trackers." Then look across the Referral Trackers box, and the second one says "Sign Ups" and if there is a number there, that means someone signed up under you!
That is if YOU wanting to know if someone signed up under YOU!
But the only way to know if you've been referred by someone else, is to ask them! Because of course, you cannot look inside their account :) Good luck! :)