one of the most famous topics is how we can know the current user credential like

user name, Groups, Authentication Type   (I hope to find a way to know password :-) )

check this code

Console.WriteLine("AuthenticationType =" + WindowsIdentity.GetCurrent().AuthenticationType);
Console.WriteLine("Name ="+WindowsIdentity.GetCurrent().Name);
Console.WriteLine("Groups :");
foreach (IdentityReference ir in WindowsIdentity.GetCurrent().Groups)
{
          Console.WriteLine(ir.Translate(typeof(NTAccount)).ToString());
}

and this a another way to get Windows Identity

WindowsPrincipal p = Thread.CurrentPrincipal as WindowsPrincipal;
(p.Identity as WindowsIdentity)

and for ASP.Net

HttpContext.Current.User.Identity

for more information
Windows Principal , Windows Identity