Well, assuming that the ancient Midnight in my XXX-S is a reasonable specimine of a stock motor:
Rarm is ~0.8 ohms, the resistance of one of the armature coils
Varm is 7.2V, the nominal voltage of a 6 cell pack.
Theoretically, V=IR, so your current is Varm/Rarm, which in this case is 9A.
For a mod motor we only need to know Rarm to do the same calculation.
Unfortunately, because of the way that motors work, that figure won't happen much in the real world.
Every time the arm turns, the electric field in one of it's 3 coils is being broken down at the same time the adjacent coil's e-field is being set up. This takes a lot wf work on the part of the incoming current, because the coil is spinning in a magnetic field, which is trying to generate a voltage in the arm counter to the one the ESC is feeding it. The coil being town down also generates a flyback voltage spike as it's contact with the brush is broken and it's field starts to collapse, which messes up the nice little DC model we use even more.
ESCs use pulse width modulation to drive the motors. In PWM, the ESC outputs a stream of pulses of power to the motor that are all at full voltage, but vary in their width (duration). The width of those pulses determines the duty cycle of the ESC, and after a little bit of filtering (those big caps we all love and/or hate, and the inherant inductance of the motor's windings) it gets turned into a relatively DC looking voltage. When you throw it all together tho, things get complicated, what with Schottky diodes and motor caps and lions and tigers and bears, oh my!
In reality, the current and voltage at the motor terminals are wildly fluxuating, all the time, and computing any kind of figure is an exercise in controlled guessing. Brush arc, contact with other cars, etc, can all cause the current to vary quite a lot. I had a 2wd off road car I instrumented a while back and measured about 9A with a stock motor, but up to 141A in short bursts, due to brush bounce and me hitting a wall. This is one of the reasons current limiters are a good thing. :-)
Now my question: Why do you want to know?