Greetings,
if you typed the name of your referred in a textbox when you created your account, you can check it here:
ucp.php?i=173 | warning thsi textbox is editable, but changing the content now has no effect!
When you wish to check who referred you by that textbox, you are out of luck, as there is no accessible overview for that.
The referral codes that you could distribute (introduced around 2-4 years ago) can track which user used it (checkable in the game client), but you cannot see the person whose code you used there afaik.