How to unit test dialog interaction

I have the following class:

public class DirectoryFinder : IDirectoryFinder
{
    public string GetDirectory(string whereTo)
    {
        FolderBrowserDialog dialog = new FolderBrowserDialog {Description = whereTo};
        DialogResult result = dialog.ShowDialog();
        return result == DialogResult.OK ? dialog.SelectedPath : string.Empty;
    }
}

How would I go about verifying that it returns the correct data? eg. string.Empty or whatever was selected in the dialog depending on what the user click?

I'm using NUnit as testing framework.