Browser Warning describes how a factory fitted wideband sensor works, and how a $20 simple electronic kit can be used to display the output on 10 LEDs.
On my Honda Insight I have chosen to fit only two air/fuel ratio monitoring LEDs to the dash - one that shows when the AFR is at 25:1 (lean cruise) and one that shows when the AFR is at 12.5:1 (full rich).
I have checked the action of the LEDs against a MoTeC PLM air/fuel ratio meter and the LEDs are always correct in their indication. In fact, they are faster to react than the tail-pipe mounted MoTeC sensor.
In short, monitoring the output of the factory fitted wideband sensor is very cheap, very simple to wire into place, and very consistent in its display. It allows people to see what is REALLY going on in terms of actual AFRs being used.