hOW we determine the session is expired in the dll of the .vb or C# file

What do you mean by "in the dll"?

Generally speaking, one easy way to see that the session has expired is to check for a value that you place in the session when it starts. If that value is missing, the session has been reset. The core problem is that you can't really check to see that a session has expired, bebause any page running will be in a good session. All you can do is check to see that the session has changed.

-Peter