ASP.Net C# Authenticate user in Active Directory

Hi Experts,
we have an ASP.Net C# webForms site, and  we have been requested to validate users in current Active Directory:
- If the user was authenticated in the AD, He or She should not see the Login Form.
- If the user did not authenticate in AD, then He or She has to see the login WebForm.

I have searched the Internet looking for working examples about it but most of them are MVC and those tutorials does not work for our development team because we are developing using WebForms. Can you please link examples or provide guidance on how to make this work?
Thanks a lot!
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José PerezAsked:
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