How to get category ids attached to a post?

What’s the best way to get all category ids that are attached to a single post?

Been playing around with get_the_terms() and wp_get_post_terms(), but just wondering if there is a simpler way that some of you plugin gurus know about?

Thanks!
-j

EDIT:

Just found out they can be accessed like this:

var_dump( $wp_object_cache->cache[ 'category' ] );
var_dump( $wp_object_cache->cache[ 'post_tag' ] );