There is a Session.GetUser( ) but unfortunately this method will also require a user GUID which you also do not know.
So there currently is no method for that, but since we are preparing a service release 3.1.7 we will add a Session.GetCurrentUser() to get the current logged in users Info record. This will require a new nuget package (will be 1.4) which will released with the 3.1.7 soon.