See the question and my original answer on StackOverflow

If you want to check an assembly is published by Microsoft, you could do something like this:

public static bool IsMicrosoftType(Type type)
    if (type == null)
        throw new ArgumentNullException("type");

    if (type.Assembly == null)
        return false;

    object[] atts = type.Assembly.GetCustomAttributes(typeof(AssemblyCompanyAttribute), true);
    if ((atts == null) || (atts.Length == 0))
        return false;

    AssemblyCompanyAttribute aca = (AssemblyCompanyAttribute)atts[0];
    return aca.Company != null && aca.Company.IndexOf("Microsoft Corporation", StringComparison.OrdinalIgnoreCase) >= 0;

It's not bullet proof as anyone could add such an AssemblyCompany attribute to a custom assembly, but it's a start. For more secure determination, you would need to check Microsoft's authenticode signature from the assembly, like what's done here: Get timestamp from Authenticode Signed files in .NET