I haven't tried it but you should be able to parallel a DC volt meter onto the connected O2 sensor wire and ground to read the signal. If it is pegged at .2V, the mixture is lean. If it is pegged at .8V, the mixture is rich. If it oscillates up and down anywhere between those voltages, you are in closed loop and the mixture is near 14.7 : 1. I don't think it will take more than a few minutes of running for the mixture to start to adapt to the new pipe. If the fuel ECU has learning and predictive capabilities, it might take a longer ride or two to get all of the possible scenarios learned in.How long should it take to adjust to a new full exhaust system?
Is there any way of changing it ourselves or playing around with it somehow?