stevez,
The thing that will prevent the motor from going any faster, i.e.: reaching its maximum rpm, is back EMF. This is caused by the motor acting as a generator and creating its own voltage. This affectively, cancels out the I/P voltage, preventing it from going any faster.
So, if you are able to find out the number of coils and the number of turns on those coils, that the motor has, with that figure and the current and voltage, you can calculate the back EMF the motor will produce. From there you can work out the speed the motor will be doing at that point, and that should give you your rpm.
Its a bit long winded and the equations to do it, are and can very quickly become more complex, but it is, in theory, possible. It depends if you can find out the number of turns and the size of the coils though, without taking the thing to bits!
Hope this was helpful to you.