Hi wonder if you can help. We have a central site which uses authO to authenticate users. this site is out of our control. when a user logs in it sets a session on the the central site and then displays a link to our site. when the user clicks the link it redirects them to our site. We cannot use any of your quickstarts because we are putting this onto an old .net web forms site. Can anyone tell me how I can get the current logged in user on our site? and then how I check to make sure they are still logged in using the authO api? and which api?
thanks 
From an high-level perspective you have two applications and in order to get the user logged in across both of them you could leverage SSO. In this case you would use the Auth0 tenant as the identity/SSO provider and the central site is already configured to use it.
In order to get the user logged in the secondary site you make a login request to the same Auth0 tenant which due to the presence of an already authenticated session could respond automatically without forcing the user to login again.
However, the issue seems to be technology stack on the secondary site; you mention .NET Web Forms, but that should not technically exclude the possibility to use OWIN middleware (Auth0 ASP.NET (OWIN) SDK Quickstarts: Login). Have you already confirmed that using OWIN would not be an option?