Sounds like what you really need is a professional penetration test, but that is cost-prohibitive. So, you need to do a pen-test yourself. Here are my suggestions for a quick & dirty pen-test. Do a good recon of your network with nmap, and see what's there. Then, use msfcli to script out some auxiliary modules. Run this script, and see what comes up. Then, as time & resources permit, script out some exploits with msfcli, and run that. You may find armitage helpful. Depending on your budget, nessus may be a good alternative, too.
Of course, there's always the Pentesting With BackTrack course at Offensive Security After taking the course, you can truly pen-test your network.