There are several ways of doing that. The easiest would be to place a shunt resistor in the ground lead and measure the voltage across that resistor. Anything that is dropped across that resistor is not available to the motor and is burnt as heat. Setting up an A/D to read that is tricky.
Here is an interesting link on hall sensors. How to measure current using Arduino and ACS712 current sensor You can increase the capacity of this sensor by paralleling its shunt resistor with an external resistor and measure much higher currents.