Expire session automatically and detect if the session has expired in Codeigniter

I've seen plenty of questions here about my topic but it seems I still haven't found my answer. What I'm actually looking for is when the session expires the user will be automatically redirected to a page saying Your session has already expired. Please register to continue browsing..

Actually I have no idea on how I could determine if the user's session has already expired. Here's what I have so far.

Thanks.

function trial() {

    $this->db->select()->from('user')->where('user_type', 'tester');
    $tester = $this->db->get();

    foreach ($tester->result() as $row) {

        $data = array(
            'id' => $row->id,
            'first_name' => $row->first_name,
            'last_name' => $row->last_name,
            'display_name' => $row->display_name,
            'email' => $row->email,
            'avatar' => $row->avatar,
            'user_type' => $row->user_type,
            'logged_in' => TRUE
        );
        $this->session->set_userdata($data);
    }

    redirect(base_url(), 'refresh');
} 

you can store data in session object and user redirect any page , you can check session object already exist if not exist you can show alert, session is expired