I have the following piece of code:

protected override void DoClose()
{
    IDisposable dispoable = enumerator as IDisposable;
    if (dispoable != null)
        dispoable.Dispose();

    dispoable = enumerable as IDisposable;
    if(dispoable != null)
        dispoable.Dispose();
}

How do I verify that it correctly disposes both enumerator and enumerable when the method is called?

Here is the test that I wrote, it is using Rhino Mocks' Multi Mocks feature to generate a proxy that has more than a single implementation.

[Test]
public void WillDisposeInternalEnumeratorAndEnumerableWhenDisposed()
{
    MockRepository mocks = new MockRepository();
    IEnumerable<Row> enumerable = mocks.DynamicMultiMock<IEnumerable<Row>>(typeof(IDisposable));
    IEnumerator<Row> enumerator = mocks.DynamicMock<IEnumerator<Row>>();
    using(mocks.Record())
    {
        SetupResult.For(enumerable.GetEnumerator()).Return(enumerator);
        enumerator.Dispose();
        ((IDisposable)enumerable).Dispose();
    }
    using (mocks.Playback())
    {
        DictionaryEnumeratorDataReader reader =
            new DictionaryEnumeratorDataReader(new Dictionary<string, Type>(), enumerable);
        reader.Dispose();
    }
}

Simple, and to the point.