How Do I Find The Session Value Is Clear Or Not ?

See more:

ASP.NET

I have to show the message alert box when session expires.
I tried like this in my Master page,

public partial class New_Admin_Master : System.Web.UI.MasterPage
{
    MessageBox msg = new MessageBox();

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            if (Session["loguname"] == null && Session["BranchId"] == null)
            {
                Response.Redirect("../Login.aspx");
                msg.ShowAlertMessage("Session Expired");
            }
        }
    }
}

it works,but i need to check the session values every second,once the session value reached null then i have to show message like("Session Expired") in my Popup box,after clicking Ok in Alert box, it should redirect me to my login page.
How do i change my code to get this result?