Using the old REST API, you could do call https://api.facebook.com/method/pages.isAdmin
to know if a user was the admin of a given page.
How can you do
$session_key = $_REQUEST['fb_sig_session_key'];//maybe there is a better way to get this
$access_token = $facebook->getAccessToken();
$is_admin = $facebook->api(array('method'=>'pages.isadmin', 'page_id'=>'YOUR_PAGE_ID', 'session_key'=>$session_key, 'access_token'=>$access_token));
From FB docs: GET
/v2.2/{page-id}/roles/{user-id}
Also see How can i get the admins user id of a page in facebook.?.
You may be able to view that info in the accounts connection
You can try this:
$page=$facebook->api('/[PAGE ID]?fields=access_token');
if (!$page['access_token'])
echo "is not admin";
Another option would be to run FQL on page_admin table.