Get array of shortcodes within string

function wpse250308_get_questions() {
    global $post;
    if ( preg_match_all('/\[question(.*?)\]/', $post->post_content, $questions ) ) {
        $questions = array_key_exists( 1 , $questions) ? $questions[1] : array();
        // $questions will contain the array of the question shortcodes
        //Do your stuff
    }
}  

$post isn’t available before wp. So, you have to hook on it or actions that fires later.