Protocol used for VPN user authentication? PAP? CHAP? EAP?

There's a security audit underway over here and a question came up regarding the protocol used for SSL VPN user authentication. We're using Active Directory, and then follow up with DUO two factor. What protocol is being used for authentication...PAP, CHAP, EAP..? I checked the VPN configuration and didn't see anything referenced/specified and my online searching so far has turned up zip. If unknown, is there an easy way to determine what we're using?

Thanks!