How Do I Find Out If My Girlfriend Has Dating Site Profiles?

You look her up.

Try going to dating sites you think she may use. Try searching for her on them by using her email address. Or you can google her name and it may bring her up as having an account with one. Google is very helpful and people don't know that when they hit accept when joining websites it is logged and shared. So much info like, Facebook, twitter, bankruptcy, any lawsuits all sorts of things. Google yourself! Good luck