Motor Voltage known but nothing else
'Rough' is right. You're going to need a multimeter. While you're waiting for that to arrive, here are a few notes.

You're right to think that you can reduce the speed by adding resistance. It's not very efficient because you waste power as heat in the resistor but for a small motor (e.g., as in a toy) it may be adequate.

Standard practice these days is to use pulse-width modulation (PWM) where full voltage is applied to the motor in short pulses and current is off for the remainder of the time. If this is done at a high enough frequency (maybe 100 times / second for your motor) the motor will run smoothly. By varying (modulating) the pulse width between 0% and 100% you can get infinitely variable speed. The efficiency is much better than the resistor as all the power used is going to the motor. (OK, a little heat is lost in the PWM controller.) Again eBay may be your friend here.