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http://weblogs.asp.net/imranbaloch/archive/2010/04/05/reading-all-users-session.aspx
How this works :
InProc session data is stored in the HttpRuntime’s internal cache in an implementation of ISessionStateItemCollection that implements ICollection. In this code, first of all i got CacheInternal Static Property of HttpRuntime class and then with the help of this object i got _entries private member which is of type ICollection. Then simply enumerate this dictionary and only take object of type System.Web.SessionState.InProcSessionState and finaly got SessionStateItemCollection for each user.
Summary :
In this article, I show you how you can get all current user Sessions. However one thing you will find when executing this code is that it will not show the current user Session which is set in the current request context because Session will be saved after all the Page Events...
To the best of my knowledge, with standard in-memory sessions you cannot. This is something I was grappling with last week and I came to the determination that it isn't possible unless you are using a session state server. Seems rather odd from a design standpoint if you ask me. :/