A brushless motor is almost like a 3-phase alternator motor.
3 coils acivated one by one to build up a rotating magnetic field to rotate a rotor with a magnet. The only difference between a BL and a normal 3 phase motor is that with BL the coils and/or sensors are used to give feedback to the ESC so the can follow the rotations at any time.

To measure the voltage on the motor you can make a 3-phase diode bridge.

And yes, BL motors will not work when directly connected to a DC power source, that will only pull and hold the rotor into one position and burn the coils.