how do i find out what email address i used when i set up my apple id

See if you can recover the ID/password. Use every email address you can think of.

Apple ID - How to find your Apple    An iCloud e-mail address is an Apple ID.

Apple ID - Find              

If you forgot the Apple ID password, enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification  email option if necessary.

Apple ID - Manage            

Apple ID – Forgot Password