I just googled around for gauges and sensors, it doesn't seem like a very practical way to go. What I had in mind was keeping the O2/CO2 mixture about the same as normal room air, which may not actually be the best way to go anyway. CO2 under a hen is probably a little higher than normal room air would be, due to the respiration of the eggs.
So I'm inclined to agree with Akane, and shoot for ways to ensure good ventilation plus correct humidity.
I've learned over the years, that the most economical way to go is usually pretty low-tech. Maybe wet sponge around the ventilation holes, (think donut) so than air will be somewhat moistened as it enters the 'bator. That way the vent holes are open, for good air movement, and incoming air will not be as dry.
A tray of water could be placed underneath, too, as log as the 'bator sits over it, and not in it. A shallow oven pan, for instance, with the 'bator resting up on the sides of the pan. Sort of like some people do with houseplants that need higher humidity, they set them on top of trays of pebbles, with water over the pebbles. The planter rests on the pebbles, not in the water. I'm thinking of my LG styrofoam 'bator, different strategies would be needed for bigger/different style 'bators.
The wet sponge inside the 'bator the last three days, too, probably.