Checking for state's existence

Where is the best place to test in asp.net apps that values placed in
state still exist (the person walking away from their browser after
logging in thing)? Putting the check in the masterpage seemed ideal, but
the page events fire first, so that isn’t a lot of help. Should I put
the validation into a base page inherited by each page? Or each page’s
Page_Load event? There is a lot of expertise here, I would like to hear
the best practices.

Thanks, Mark

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