In my case trying to do the same i send the session id as part of the authentication validation.
SessionToken = p_web.SessionID
Then before call any NetWebServiceMethod i set the SessioID i validate if it is logged in
p_web.SessionID = SessionToken
IF p_web.GetSessionLoggedIn() = 1
Some Code
ELSE
p_web.AddServiceError(1,'WebServiceMethodCall','','LoggedIn','user Not Logged In')
END
Don't know yet if there's a better way but it works for me.