session timeout

I ahve a data entry site where users session variables are saved at login.  If they sit idle, the session variables timeout.  I want to redirect to a certain page on the timeout of these session variables.  Is there a way to do this in the default.master file?  Could I use an if statement on every postback? 

like this maybe? 

on postback

{

          if (Session("GUser") == null)

{

           response.redirect(timeout.aspx)

}

}

 

 

Thanks in advance
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