What are you trying to do?
There is no event just preceeding Session_End which would you can use to access the Session object.
If you stored a action timestamp relating to the last time the Session object was used, and then checked that, you could get some idea of when it should end.
(Based on your IIS settings / configuration).
However the act of accessing it would prolong the life of the Session object.
Of course you'd need to check this somehow, either by using a hidden Ajax postback which fires an event or something similiar.
But really I wouldn't advise this. What are you trying to do that you need this?