Session variable assingment issue

One thing that I've found useful to do in my apps (corny though it may sound) is to define a variable "Session.exists = 'yes'."

Then, at the top of all of the relevant pages, I check (using StructKeyExists) to make sure that the Session object has an "exists" member.  If it does not, then I know that either the session has not been started, or it has expired.  And I redirect the user someplace to handle it.

That "someplace" is a cfm that the user ordinarily would never see.  And before I go there, I set another dummy value (say, "Session.foo") in some session-variable.  Thus, when we arrive at that "someplace":

1. If we are here, then it's because the session expired or could not be established.
2. If we are here, and Session.foo does exist, then it was an ordinary timeout.  We delete the Session.foo variable, then redirect to a login screen with an appropriate message.
3. If we are here, but Session.foo does not exist, then there's something wrong with session-management on the client computer and we redirect with a "cookies are required" message.

All of this can be neatly handled in the boilerplate ("write it and fuhgeddaboudit") prologue of every-page, i.e. a cfincluded member.

(Nope, I don't have code to post.)

Do all session variables fail or just certain one(s)?