The user doesn't get redirected to Myspace, it is just a pop up window so he never leaves your site. There isn't a way for you to know if he shared anything, but you could use the rest api here instead (see near the bottom for the Share example): http://wiki.developer.myspace.com/index.php?title=Status_and_Mood_API
POSTing a status/mood with this API is the equivalent of sharing a status mood. We have API's like Mediaitems for which you can post images and videos: http://wiki.developer.myspace.com/index.php?title=Media_Items_API
There isn't an api equivalent which can do exactly what Share does, however.