This might be useful for debugging purposes. Add the following code to AfterAppInit event (C#) to do so.
for (int i = 0; i < HttpContext.Current.Session.Count; i++)
{
var crntSession = HttpContext.Current.Session.Keys[i];
HttpContext.Current.Response.Write(string.Concat(crntSession, "=", HttpContext.Current.Session[crntSession]) + "<br />");
}