Testing Motor for Specs?

[indieFan]

I'm working on my senior design project and have gotten motors out of a toy truck that I need to determine the specs for.

I already know I can use a tachometer or strobe light for getting the rpm of a motor. However, how do I determine the max voltage that the motor can take so that I know that I'm looking at the correct rpm and overall voltage requirements of each motor? Also, how would you physically set this up using multimeters and a voltage source? (I'm assuming that I'd have to use a voltage divider if the voltage source is a constant voltage. i.e., 12V.) Can I use one of the charged batteries for a FIRST robot to provide the power source or am I looking at a dangerous situation?

Any help would be appreciated.

Thanks,
indieFan