If you have a diagnostics kit capable of logging details, hook your laptop up and set it to record the readings from both lambdas the throttle position and the fuel trims. Take the car for a spin and give it some different driving conditions (accerleration, deceleration and drive at a constant speed for a bit should do the trick).
It will then be obvious if you have a faulty lambda.
Your readings on your lambdas should reflect the amount of fuel put in when things change. So when you floor it the lambda readings should shoot up until the computer can bring it down with the short term fuel trim, and when you let off the throttle and let the engine slow the car down they should drop to near 0. Also at constant speed the ecu should be able to control the fuel and emisions the same way as at idle so the first sensor will fluctuate between 50mv and 950mv.
Even with mods that aren't accounted for in the maps on the ecu it should still be able to control the fueling and thus alter the sensor readings as it does normally. The worst that should happen is your engine wont run at full potential.