I did this to confirm my 99 cat was not holding me back, should work for you:

Pull out the front O2 sensor on side you feel is suspect. Get a fitting with the same thread as the o2 sensor (I forget what it was) one side - small barbed nipple other side, screw it in the O2 bung, and run a 10' section of small tubing from the barbed fitting to a vacuum gauge in the cab...go for a ride and run it up. If you have more then 2-3lbs of back pressure the cat is restricted.

You could always compare the back pressure reading to the other side if its a close call.

Cheers!