Can’t get post_updated hook to work

If it doesn’t work you can try to use a wp_insert_post action:

function insert_post_hook($post_id, $post) {

    if ($post->post_date != $post->post_modified) {
        // This post is being updated!
    }
    else {
        // This post is being created!
    }
}
add_action( 'wp_insert_post', 'insert_post_hook', 10, 2 );

If you want to see the latest version you could enable revisions:

define('WP_POST_REVISIONS', 3 ); 

This will save a maximum of 3 revisions.

After enabling you can check the following:

$latest_revision = array_shift(wp_get_post_revisions($post->ID));