You need to write a little PHP code. When user first click tab you can check is he like the page or not. Below is the sample code
include_once("facebook.php");
// Create our Application instance.
$facebook = new Facebook(array(
'appId' => FACEBOOK_APP_ID,
'secret' => FACEBOOK_SECRET,
'cookie' => true,
));
$signed_request = $facebook->getSignedRequest();
// Return you the Page like status
$like_status = $signed_request["page"]["liked"];
if($like_status)
{
echo 'User Liked the page';
// Place some content you wanna show to user
}else{
echo 'User do not liked the page';
// Place some content that encourage user to like the page
}