how to check session is expired or not if expired then redirect to login page

protected void Page_Load()
{
if (Context.Session != null)
{
if (Session.IsNewSession)
{
string cookieHeader = Request.Headers["Cookie"];
if ((null != cookieHeader) && (cookieHeader.IndexOf("ASP.NET_SessionId") >= 0))
{
Response.Redirect("sessionTimeout.htm");
}
}
}
}