If you want to know how much traffic is sent over a specific interface at a moment in time, you have several ways. The first one is to set the 'load-interval' under the interface to 30s (load-interval 30) and check the 'output rate' counter under show interface. This only shows a 30s average but it gives you an idea of the current interface utilization. Another way to do this would be to take snapshots of the 'output byte' counters under show interface at regular intervals and make output rate averages across those intervals. This could be done manually or through the use of proper SNMP get commands. Some management software could be be used such as MRTG.
For more information, you could have a look to the following URL:
http://www.cisco.com/warp/customer/121/pvc_utiliz.html