How to check post meta isset or not empty during publish/save post?

I don't know what you want to do besides checking the meta so i'm gonna give a very basic example.

function bt_check_saved_post_meta ($post_id) {
    if (!empty(get_post_meta($post_id, 'some_meta', true))) {
        // do something here
    }
}
add_action('save_post', 'bt_check_saved_post_meta');