Hey Mike,
SNMP offers MANY parameters for discovering faults on Cisco devices.
You may want to research the Cisco web site for a SNMP guide for your hardware/software.
Debug is not normally run full-time in a production environment due to the high overhead.
Checking log files is a good way to be proactive although this will become VERY time consuming as the environment grows.
If SNMP is not providing you all the information you desire, perhaps you may want to take a look at Cisco's Netflow for additional features.
One word of caution - SNMP and Netflow can be VERY time consuming. Most larger shops hire a full-time person (2) dedicated to each of these technologies.
HTH
Frank