How do I prevent my users from accessing directly pages meant for ajax calls only?

Passing a key during ajax call seems like a solution, whereas access without the key will not be processed. But it is also easy to fabricate the key, no? Curse of View Source...

p/s: Using Apache as webserver.

EDIT: To answer why, I have jQuery ui-tabs in my index.php, and inside those tabs are forms with scripts, which won't work if they're accessed directly. Why a user would want to do that, I don't know, I just figure I'd be more user friendly by preventing direct access to forms without validation scripts.