If one doesn't have the source code, the only to test is to send keystroke and mouse move so I found this product but it's not free and opensource price is not even known :(
http://doc.froglogic.com/squish/4.1/all/ugs-webnonhtml.html
Squish supports automating interactions and testing non-HTML/DOM elements, that is, native objects, which are embedded in a web page. This is done at a fairly abstract level, which means that mouse and text input can be recorded and replayed. In addition it is possible to inspect embedded native objects with the Spy tool and to insert verifications for these native objects. All of a native object's public properties can be accessed in test scripts.
You could look at Sikuli IDE .
It is more geared for automation than for testing, but it works nice.