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If you have a narrow band O2 sensor the LED light air:fuel ratio gauges will tell you when you are in closed loop or not. If the car is in closed loop, the LEDs will oscillate about the central light. If not, they will be steady either rich or lean.
There's no real functional difference between a gauge bought assembled, one assembled from a kit of parts or one bespoke built from one of the better designs found on the interwebs.
If you have a wide band sensor a volt meter should do it. AFAIK they put out a 5V signal varying with the air:fuel rati, ~2.5V being stoichiometric ("closed" loop).
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