at 2:49 in the video, he shows you how to read the amperage being sent to the motors, you should also know what amperage your motors are designed for. (in this case, i believe he said they are rated for 1.5a)
You want to start out with your potentiometer at it's lowest setting, then power the device, as he has done, and turn the potentiometer up until you reach this amperage. Going beyond a tiny bit may not cause damage, due to tollerance allowances by the manufacturer of your motor (i.e. they often undervalue the device for safety reasons) However, going above the manufacturer's suggested limits is asking for trouble.