Oh, so you're using the Specific Post/Page restriction?
I know that the custom value is there as well, so it should be available when the person uses the special URL to view the page he paid access for. I'm not exactly sure where you'd find it, it'd take a little bit of testing to find out.
For logged in users, there's a constant s2Member makes available, but I'm not sure that the same applies to a not logged-in user while using the special access URL... WP Admin > s2Member > API / Scripting > s2Member > PHP/API Constants >
You could start your tests using this on the page and viewing it with the access your user would have:
<?php print_r(get_defined_vars()); ?>
I hope that helps.