I used to calibrate industrial gas detectors. They generally have an active air sampling path with a pump and a sensor in the gas flow path, and we would calibrate them by flooding the sensor chamber with a test gas at a known concentration. For the type of sensor you have none of those conditions exist (no controlled gas flow path and no pump) so there is essentially no way you can accurately test their response. You essentially have to trust the manufacturer to have made a good product. The one thing you can do is look for an install date and (hopefully) an expiration date, if you can inspect the back of the detector without setting off any central alarm system. If you really don’t trust the owner’s sensors, you can always buy and use your own. Just make sure you place them carefully according to the manufacturer’s instructions.

Also, your detectors are probably combo smoke/CO, but not natural gas. You would smell the odorant in the gas long before the concentration becomes dangerous so a detector would be redundant.

It doesn’t, but if I recall correctly there were a lot of owners complaining that the brushed finish would discolor if you touched it with your hand. Body oils and salt left finger marks everywhere from people touching the car.