I can check shares like this
https://graph.facebook.com/?id=http://amazon.com
this will give me a response like
{
"id": "http://amazon.com",
"shares": 1395576,
"comments": 14
}
Now, after a user has clicked on share button I can again call
https://graph.facebook.com/?id=http://amazon.com
and if share count has increased I will know it has been shared. Is there any other better way?