Trying To Detect Session State Timeout?

I am trying to detect a session state timeout in my asp.net application and am unable to do so. I have a base class that derives from System.Web.UI.Page as follows:-

public class BasePageSessionExpire : Page
{
override protected void OnInit(EventArgs e)
{
base.OnInit(e);
if (Context.Session != null)
{
if (Session.IsNewSession)
{
string szCookieHeader = Request.Headers["Cookie"];
if ((null != szCookieHeader) && (szCookieHeader.IndexOf("ASP.NET_SessionId") > 0))
{
Session.Abandon();
Response.Redirect("~/SessionExpired.aspx",true);
}
}
}
}
}

All the pages I need session state checking on derive from this base class instead of "System.Web.UI.Page". Also, all these pages have EnableSessionState="True". I have a blank Session_Start() method in my global.asax file if that is relevant at all.

For some reason after the first request, the "Session.IsNewSession" property is always false. It is true only for the first request and then is always false. I have my timeout set to 1 minute. The session never seems to timeout. What am I missing here ?

Also I have implemented a state server in SQL Server 2008. This is not an in-proc session state implementation.