show logged in users

hello anohter question: whats the best way to see which users are logged in? - reading out the pppoe router with snmp for active sessions - search for session ids with start but no stop event i want do the first one. becuase our routers are in isolated management network and i want access the router from our webinterface or some admin hosts.... can i be 100% sure that a user is logged in if there is no stop event? thanks and kind regards marco fretz