Detect Session Timeout

To detect session time out in asp.net, you can add a config to asp.net as shown below.

<authentication mode="Forms">
  <forms loginUrl="Login.aspx" protection="All" timeout="30">
  </forms>
</authentication>

or you can check your session as the following

if(Session["your_key"] != null)
{
   //Session is not expired
}
else
{
   //Session is expired
}

You can also use this way to detect session has expired or not.

protected void Page_Init(object sender, EventArgs e)
    {
        if (Context.Session != null)
        {
            if (Session.IsNewSession)
            {
                HttpCookie newSessionIdCookie = Request.Cookies["ASP.NET_SessionId"];
                if (newSessionIdCookie != null)
                {
                    string newSessionIdCookieValue = newSessionIdCookie.Value;
                    if (newSessionIdCookieValue != string.Empty)
                    {
                        //This means session was timed out and new Session was started
                        Response.Redirect("Login.aspx");
                    }
                }
            }
        }
    }