I have good solution for this....
Encode your url to current datetime using and then add this url into session object e.g.
Session("update")=Server.URLEncode(System.DateTime.Now.ToString())
Add this session to view state. For this, you can use page pre_render event
Sub Page_PreRender (sender As Object, e As EventArgs)
ViewState("update") = Session("update")
End Sub
and finally check this session to view state
If (Session("update").ToString() == ViewState("update").ToString())
{
}