Check the metabox global directly?
$wp_meta_boxes['YOURTYPE'] should hold data on metaboxes for a given type.
Had a quick look in wp-admin/includes/template.php but i can’t see any convenience functions for extracting metabox information from the array so you’ll probably need to create your own code to loop over or extract the necessary info from it.
Of course make sure you run your code after the add_meta_boxes actions(else they’ll likely not exist at the point you run your code).