Router Exploit

I have a couple of trendnet routers on officies outside the company. I have configured those to send me e-mails about suspicious activity (like DOS attack or port-scaning). From the every day report I see that some outsiders have found the open ports, which are some port-forwarding rules for the LAN behind these routers.

Now, I want to know if there is any way to find out if these outsiders have succesfully passed the password barriers and have managed to enter the LAN through these open ports? It's not dificult to find out an open port on a network, but after that can they come through? What should I use to see if there was a succesful exploit?