Hi,
You can check the traffic rate by looking at the interface counters with the 'show interface' command. You might also want to check all interface rates at once with a single command, you can use the 'show interface | include rate' (short: sh int | in rate) command to print the average traffic rate for all interfaces.
If you want to change the period of time on which the average calculation is based, you can use the 'load-interval ' interface configuration command. The minimum value is 30 seconds.
You can also use the 'show controllers utilization' command to display bandwidth utilization on the switch ports.
Refer to the following link for more information:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_44_se/command/reference/cli2.html#wp3419973
Andras