How do I know if a class is a wrapper for an unmanaged resource

How do I know if a class in C# is unmanaged, so that if I use it in a self defined class I know whether I have to implement the IDisposable interface?

If I get this article on the MSDN network right, I always have to implement the IDisposible interface when I use an unmanaged resource.

So I've created a little example that you can find below:

class TestClass
{
    private StreamReader reader;

    public UsingTestClass()
    {
        reader = File.OpenText("C:\\temp\\test.txt");

        string s;
        while (!string.IsNullOrEmpty(s = reader.ReadLine()))
        {
            Console.WriteLine(s);
        }

    }
}

The following MSDN article says for example that File is an unamanged ressource, which is also used in my test class. So how can I see, that this class is unamanged? The compiler and ReSharper didn't complain anything.

Thanks in advance.