When paying with PayPal How do I get information (Silverlight Web)

Silverlight project with the code below into the pay section, but insufficient

How can I see that the payment successfully?

            HtmlPage.Window.Navigate(new Uri("https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=cengiz7karaaslan-facilitator@gmail.com&item_name=Les Arts&amount=" + _Price.ToString().Replace(',', '.') + "&return=http://localhost:59252/Default.aspx"), "_blank");