code for checking whether a session exists or not

The session will always exist probably because when your session expires and you access a page again it just makes up a new session for you.

One thing you can do is the first time you access the app just set a session variable like:

Session["LoggedIn"] = "1";

Then check to see if it exists. If it doesn't then you know the session has been expired and recreated so you can boot them back.

if (Session["LoggedIn"] == null)
{
Response.Redirect("YourSignInPage.aspx");
}
else
{
// Do whatever you were going to do.
}