How to know user has authenticated in asp. Net?

For Is Postback refer
1) https://www.codeproject.com/Articles/811684/Understanding-The-Complete-Story-of-Postback-in-AS
2) http://net-informations.com/faq/asp/ispostback.htm

For Authenticated in Asp.Net refer
1) https://www.codeproject.com/Articles/98950/ASP-NET-authentication-and-authorization
2) https://msdn.microsoft.com/en-us/library/xdt4thhy.aspx
3) https://www.c-sharpcorner.com/article/authentication-and-authorization-in-Asp-Net/