I'm assuming your web application is set up for Anonymous (no) authentication so that would require that the user supply his/her account ID. With that information you can check to see if they are in a particular domain.
You can use the System.DirectoryServices namespace to do this. Probably the easiest method would be to use the WinNT provider. The LDAP provider would require a bit more code.
Dim ADEntry As New System.DirectoryServices.DirectoryEntry("WinNT://<domain name>/<account ID>")
Catch ex As Exception
Console.WriteLine("User not found in domain")
Microsoft MVP (Visual Basic)