Asynchronous method testing in C#

I have the following event handler:

private void source_load_complete()
    {
        Dispatcher.BeginInvoke( new NotificationDelegate( source_load_complete_ui ), null );
    }

and then:

private void source_load_complete_ui()
    {
        m_image.Image = m_bmp.CreateBitmap();
        m_image.UpdateImage( null );
        m_image.LoadComplete = true;
        raise_property_changed( "CurrentImage" );
    }

And to test this, the following test method:

[TestMethod]
    public void ImageVM_SourceLoadTests()
    {
        ImageViewModel ivm = new ImageViewModel();
        List<string> eventList = new List<string>();

        ivm.PropertyChanged += delegate( object sender, System.ComponentModel.PropertyChangedEventArgs e )
        {
            eventList.Add( e.PropertyName );
        };

        // check the model is set up correctly so the event in the constructor works
        CustomImage model = ivm.ImageModel as CustomImage;
        WriteableBitmap bmp = model.Image;

        if( model.ImageSource.LoadComplete != null )
        {
// fire off the event we want to test is handled correctly
            model.ImageSource.LoadComplete();
        }

        Assert.IsTrue( model.LoadComplete == true);
        Assert.IsTrue( model.Image == bmp );
        Assert.IsTrue( eventList.Count == 1 );
    }

Clearly this will not work, as the source_load_complete_ui method is invoked asyncronously. However I can't figure out after searching around how best to test this and wait for the asynchronous method to be called?