https://graph.facebook.com/me/likes/PAGE_ID
&access_token=ACCESS_TOKEN

<?php
require '../src/facebook.php';
 
// Create our Application instance (replace this with your appId and secret).
$facebook = new Facebook(array(
  'appId' => 'APP_ID',
  'secret' => 'APP_SECRET',
));
 
$user = $facebook->getUser();
 
if ($user) {
  try {
    $likes = $facebook->api("/me/likes/PAGE_ID");
    if( !empty($likes['data']) )
        echo "I like!";
    else
        echo "not a fan!";
  } catch (FacebookApiException $e) {
    error_log($e);
    $user = null;
  }
}
 
if ($user) {
  $logoutUrl = $facebook->getLogoutUrl();
} else {
  $loginUrl = $facebook->getLoginUrl(array(
    'scope' => 'user_likes'
  ));
}
 
// rest of code here
?>

FB.api('/me/likes/PAGE_ID',function(response) {
    if( response.data ) {
        if( !isEmpty(response.data) )
            alert('You are a fan!');
        else
            alert('Not a fan!');
    } else {
        alert('ERROR!');
    }
});
 
// function to check for an empty object
function isEmpty(obj) {
    for(var prop in obj) {
        if(obj.hasOwnProperty(prop))
            return false;
    }
 
    return true;
}

$result = $facebook->api(array(
    "method"    => "fql.query",
    "query"     => "SELECT uid FROM page_fan WHERE uid=$user_id AND page_id=$page_id"
));
if(count($result))
    echo "$user_id is a fan!";