prevent proceed opening php page

how i hinder users accessing directly pages meant ajax calls only?

passing pivotal during ajax seems solution, since opening but pivotal processed. nonetheless also easy heed key, no? abuse perspective source...

p/s: controlling apache webserver.

edit: answer why, i have jquery ui-tabs index.php, inside those tabs forms scripts, won't work they're accessed directly. since user wish that, i don't know, i only figure i'd some-more user permitted preventing proceed opening forms but validation scripts.