How can you tell if a session has expired

How can you tell if a users session has expired based on the session time out
in the <cfapplication> tag?

<cfapplication name = "test"
clientmanagement = "Yes"
sessionmanagement = "Yes"
setclientcookies = "Yes"
sessiontimeout = "#CreateTimeSpan(0, 0, 3, 0)#"
applicationtimeout = "#CreateTimeSpan(0, 0, 3, 0)#">

A user logs into the site, pulls up some page and then doesn't come back to
that page for 10 minutes. Is there a way to direct this user to a session time
out page? It seems that once a user times out, you don't know if that user has
already been to the site or if they are a new user hitting the site for the
first time.

I basically want a user who has previously logged into the site, and who has
timed out, be redirected to a session time out page, but a new user would be
directed to a login page. I just can't figure out how to determine if a user
has logged in and timed out versus a new login.

A couple of layers can be applied to this.

On the server you can put code into place that detects if a given
session variable is defined on pages that only logged in users can view.
If this variable is undefined that was created when the user logged
in, then redirect the request to your session expired page.

You can combine this with a javascript layer on the client that uses a
timer set to about the same as the session timeout that will redirect
the user upon the specified time. As long as the user has JavaScript
enabled of course.

At time out, it is difficult to use session variables (not discussing cookies
here) to differentiate between a logged-in user and others. All your session
variables will time out if not kept alive at this point.
It is advisable to screen out users who are not logged-in from sensitive
pages. This makes our task manageable.
Now to the task!

Option 1
Put the session check and redirection at the top of the template. In this
case, users who are not logged in, as well as users whose sessions have timed
out, will be redirected to the login page.
<cfif not isDefined("session.memberId")>
<cflocation url="loginPage.cfm" />
<cfabort /> <!--- This line is optional --->
</cfif>


Option 2
Allow only logged-in users to get to the specified pages.
When the session time-out kicks in, say after 3 minutes, any action on the
page will trigger the getAlert(). The function asks the user to click OK to
keep the session alive. If the user clicks cancel, he/she will be redirected to
the login page (or as desired).
As stated earlier by Ian, set the timer - setTimeout() ? to about same time
as the session timeout period.

<script type="text/javascript">
<!--
function getAlert() {
var x=window.confirm('Your session has timed out. Please click OK to keep the
session alive')
if (x) {
if (typeof(TimeOutID) == 'number') {
clearTimeout(TimeOutID);
}
TimeOutID = setTimeout('getAlert()', 180000);
}
else
window.location='loginPage.cfm'
}
//-->
</script>
<cfif not isDefined("session.memberId")>
<script language="JavaScript">
getAlert();
</script>
</cfif>