I have used my Facebook account and Gmail account to access dozens of sites.
How can I find out on which sites I've used these authentication methods?
You are asking two similar questions. My answer will try and address both.
To see which sites have used Google Authentication:
To see which sites have used the Facebook Authentication:
More info: Facebook uses a variant of the OAuth2.0 protocol. Google+ Sign-In is built on the OAuth 2.0 and OpenID Connect protocols.