What Peteski said... with the exception of Faulhaber, the numbers on coreless cases like Maxon are proprietary numbers for industrial clients who bought a large lot of motors, and that motor you are holding in your hand is a surplus from that lot.
Get the size (i.e. 10mm diam, 24mm length), that will get you the right motor (in Maxon's case, usually an "RE10 24" for that
size). The terminal resistance will get you the nominal max voltage because the data sheets usually list the terminal resistance
or the max current, which you can use to compute the resistance with V=IR. It won't be exact, but it will be close enough to
narrow it down to only one motor.
Another way to do it, if you have one of those handy optical RPM meters (they are on eBay
for $20 - $30), measure the RPM at 3v and then look at the data sheet for the motor. You can pretty easily see what voltage motor you have. If the max RPM is listed as 12,000, and at 3v you are only at 5,000, then increase to 6v and check the RPM again. As above with voltage and resistance, you can quickly narrow down what motor you have.
I've done it both ways.
