My daughter was doing a similar experiment with the Beaman type motor. After reading the above and doing similar research I stumbled upon another method for measurement. My son had a snap together electronic experiment kit that came with a probe that plugs into the mic on your PC. The software converts the signal into something resembling an oscilloscope, which can easily be used to measure frequency of the motor.
I have some technically background and familiarity with this kind of stuff, but even if you don't, with a little research you could probably figure it out. There are several programs that can do this, I used Zeloscope, free 14 day trial, enough for the project, but only $10 to purchase anyway. The probe that plugs into your mic input is just alligator clips with a 10 M Ohm resistor inline (can't plug signals directly into the mic input, it is way too sensitive for most applications that would be measuring voltage, such as this one).
Just hook up the probe leads across the motor contacts. On the beakman motor, each revolution causes a drop in the voltage as the coil conducts, so you need to measure the timing of one full cycle to get the frequency of the motor. The wave won't be anything uniform, but that's ok. Eg, if one complete cycle lasts 100ms, you have a frequency of 10 cycles/sec (hz).
You'll have to play around a bit with the settings to get the wave to fit on the screen, so if you don't have and experience using a scope, it might be a little intimidating at first, but with a little research, you can do it (as a friend remarked, "Science fair projects really show how much the family learns
It took me a while to figure it all out (and my tech skills are a little rusty), but in the end, I think it was less time consuming than the other approaches (at least for me), a little fun, and cost nothing (I had the probe already, but it can be easily made for a couple dollars).
Hope that helps.