Nope, not possible
The callback we get from Facebook after a comment is created contains the following:
{
href: "", /* Open Graph URL of the Comment Plugin */
commentID: "", /* The commentID of the new comment */
}To my knowledge, there's no way to use that information to see if the user posted it to their wall. It may be possible to pull a feed of their wall posts, scan through them, and check for the same URL or something, but it wouldn't be a simple thing. You'd also need the permissions to see their wall and other things.
I believe Facebook wouldn't ever allow you to detect that because it could lead to some abuse for rewarding users for leaving (and posting) comments.
For more on what's returned, check out the page below. Specifically, the comment.create function:
developers.facebook.com/docs/reference/j.../FB.Event.subscribe/
Hope that helps,
Alex
AUP points on facebook comment only if select "post to facebook"
