For the longest time I’ve been waiting from WordPress to build this function, but they seem to be busy building awesome things like Custom Post Types, Menus & other Admin features! Truthfully, I should just write the function myself and submit it! I’ve always been afraid to submit to the core.
Anyway, here is a function called has_children. It checks if any posts are using the given post_id as post_parent. There is a post_type paramater so that you can be specific.