Kegelcizer

public static bool IsInAdminGroup(string userName = null)
{
    using (var d = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer"))
    using (var g = d.Children.Find("Administrators", "group"))
    {
        return (g.Invoke("Members", null) as IEnumerable)
            .Cast<object>()
            .Select(m => new DirectoryEntry(m))
            .Any(m => m.Name.Equals(userName ?? Environment.UserName));
    }