How to check if recipient accepted Facebook Game Request sender-side only?

Question

I want to reward users who invite friends into the game. I want to keep everything sender-side only, without any involvement of back-end server. After each sent request, I can store request ID and recipients list.

Once a recipient accepts Game Request, I call delete method on it, so I thought I could somehow check if the request is deleted or not. But what if an user invites 10 friends instead of 1? I am not sure how that deletion works.

What could be a good solution?