It would take some PHP code to do it and would require that you ask every user that signs up to your site grant the user_friends permission, which specifically lets your app see their friends that have also authenticated on your site. With that, you could run some code to check if they are your friend (or vice versa).
We could probably give you a code snippet that should do what you're looking for, but implementing it and 'doing' something with that knowledge would have to be done by yourself or a developer. Also, the 'user_friends' permission is something the user can choose to deny, which means that the user would be registered but they wouldn't look like a friend, even if they were. So, it could be a bit trigger and result in some false-negatives.
For more information on getting friend information, see:
developers.facebook.com/docs/graph-api/r...ce/v2.4/user/friends
I hope that helps explain,
Alex
User is FB-Friend
