Hello, Adam.
If you are using our Micro Metal Gearmotors, the motors should be stamped with some markings that we can use to help narrow down the voltage and power level. Unfortunately, the gearbox is harder to identify. Generally, the easiest way to identify the gear ratio once you know the voltage and power level is to get a measurement of its free-run speed and comparing that to our specifications for reach gear ratio. A tachometer or encoder can be used to get this measurement, but if you do not have either, you can run it at a lower voltage and extrapolate up. You can find more information about this process in this post by Patrick.
Brandon