How do I check if a logged in user is active in the mid of the website execution?

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.NET2.0

I have logged in as gsam and i am in the edit page details of product . now suddenly if the user session is expired, i would like to notify the user that the session has expired and need to login again.

Also the user should be able to store the datas until where ever he /she has entered the product details.

What I have tried:

I have checked like this in edit product details page.

In Pageload of edit product details
--------------------------------------
if (Session["User"] != null)
{
//continue with user editing the product details.
}
else
{
//Message box showing that the user expired and need to login again to edit the product details.
//also notify the user that wherever you have entered the details will be stored.
}