If you have a multimeter that can read a couple of amps or so, you can put it in series with the motor to find the current it takes. The worst case is the stall current, i.e. the current when the motor is prevented from rotating.

Alternatively, measure the resistance of the motor with multimeter. Divide that resistance into the voltage you want to run the motor from to get the stall current.

If the current is not more than 1A, you can use a 1N400x diode where x is any digit from 1 to 7. Actually, these diodes are probably good for 2A of motor current, because they are rated at 3A pulsed and they only have to take short pulses in this application.

If the motor current is higher, then go for a higher-rated diode as appropriate.