You CAN fry the diodes in the alternator or some of the other electronics in the vehicle if you pull the battery cable because the voltage will overshoot until the regulator can pull down the alternator output, which takes a small amount of time while the field collapses in the rotor. If you have the headlights on it is less likely to happen because the power has some place to go. It is still a bad idea. It may not hurt anything immediately, but it will shorten the life of something.
Buy a DMM, they cost less than $10. Much cheaper than replacing your alternator.
