Calculating electrical limits of a DC motor?
Start with the voltage - if you removed it from the toy what was the voltage being used to turn it? 

With that voltage measure the no load speed and current (see below)



As a rough guide small DC motors turn at around 1000 rpm per volt.

(The next bit is where it gets a tricky without proper test equipment). You need to load (add a breaking torque) to the motor and measure the results. By any convenient means, keeping the voltage to the motor the same, slow the speed of rotation by about 20 %. Measure the current at this speed. 

This will give you a rough idea of how the motor current/speed will change when loaded. (remember the current is proportional to torque).

I don't recommend a stall current test - this tends to burn the motor out.