[Al Skierkiewicz]

Indie,
Dave and Kevin have given you some good info so far. There is a way you can easily back into the calculation if you can partially disassemble the motor. The wire in the armature is designed to sustain full stall current. If you can measure the diameter of the wire, throw in a little fudge factor and then take a look at a wire size versus current handling table, you will get a rough idea of the design for the stall current maximum. Say, that from the above you find the wire in the motor is about 25% larger than a wire that would handle 10 amps. Making the assumption that stall current is then 10 amps, you need a variable voltage source connected to the motor through an ammeter and switch. Start out with a voltage that is lower than the battery in the toy, stall the motor shaft and momentarily supply a voltage to the motor while observing the current. If it is close to 10 amps then the battery voltage is most likely the design voltage for that motor. If it turns out to be significantly higher, then lower the voltage and try again. A simple variable voltage supply is a "D" cell battery holder and a couple of test leads. Each battery is 1.5 volts, so 8 in series equals 12 volts, 7=10.5v, 6=9v, etc. Once you know the design voltage and stall current you can determine the no-load speed at the design voltage. As to the other parameters you can really study the motor curves for robot motors and attempt to put enough mechanical resistance on the motor to change output speed while monitoring current. With this data in hand you can make some educated guesses about the motor specs without needing to use all of the sophisticated equipment that motor designers use to check their designs.
Now the hint, if the motors have any markings on them at all, most manufacturers will build into the part number the operating voltage range. The common ones for toys being 12, 10, 6 for lead acid batteries, 12, 9, 6, 3, 1.5 for alkaline batteries, 12, 7.2, 3.6, 2.4 for rechargeable NiCads. However, a toy manufacturer may not try to stay within the design specs of the motor. They may for example be perfectly happy to run a 9 volt motor at 6 volts, not worrying about full stall current or speed. It allows them to reduce the size of the wire feeding the motor and back down on the transmission design by using the tradeoffs of a lower supply.