How do I know if an user did shared a facebook link?

how do i check if the user really shared a link? for example i want to give some coins (or something) to a player whenver they share their score, i already did all the score system and the user can share it on his facebook wall, the question is, how do i know if he really shared it? i dont see any event or action that let me check if the player really posted that in their facebook, cause when they press the share button, its appear the facebook interface with the "share" button, but they can close it.

I would say that you would probably have to a value which saves on their device.

I think it would be done by adding the local storage in to your game

then add a action

Local Storage>Set Item

Key is the name of the file

eg FBShared

Then Value I would do 1

1 = Shared

If the player has not shared it there will be no storage, so when checking when they press share, do check item exists.

I would show screen shots explaining it as I am a bad person to explain things, but I don't know how (Yes i do know print screen, but you can't just paste it in)

Good Luck!