Facebook Graph API - likes returns me an empty set

I read in the Changelog of 2010-12-12:

GET [page-id]/members/[user-id] will return the individual user if he or she is a fan of the page (Graph API equivalent of pages.isFan) (rE322057)

So, you can check with Graph API if a known user_id is fan of a known page_id with exactly this syntax. You need to supply a valid access_token. With the fields parameter you can choose, what the API should return. E.g. append "&fields=id,name,picture" to get the id, name and picture in case the user is a fan.

{
   "data": [
      {
         "id": "712084216",
         "name": "Mike Bretz",
         "picture": "http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs842.snc4/70441_712084216_5491098_q.jpg"
      }
   ]
}

You'll get an empty result if the user is not a fan

{
   "data": [

   ]
}

add user_likes permission to your application