How do I detect outgoing spam?

I’m running the network for a small business on an SBS2003 server, using a cable modem and a NetGear RP614 switch/firewall. Is there an inexpensive way to see if any of my (20) clients are inadvertantly spamming (like looking for suspiciously high smtp packets)? I keep everything and everyone fully patched, but from time-to-time a user will disable their AV or spy-detection software (we can’t afford a global policy-based solution).