If you are using "InProc" mode of session handling, then SessionID id can be stored in 2 places based on the configuration in "Web.Config".
1) <sessionState mode="InProc" cookieless="true"> </sessionState>
In the above config line, we set "cookieless=true", at this case the session id will be stored in URL itself. So the user can easily view the session id, which is vulnerable one.
Ex: http://localhost:57913/(S(5a1v1hngmwbfn54wl5jdlmp3))/WebForm1.aspx
2) <sessionState mode="InProc" cookieless="false"> </sessionState>
As per the above line, the session id will be stored in Cookie with the name "ASP.NET_SessionId".