Electric potential is also known as voltage, which is measured in volts.
Voltage (which measures the work done per charge) is calculated by the formula
Electric potential difference is usually measured using a voltmeter, which is placed around the object (in parallel) causing the electric potential difference. This could be around a cell, showing the increase in voltage, or around a resistor, showing the drop in voltage.
It is only differences in potential that can be measured. As with all forms of potential energy a reference point or ground needs to be specified in order to make the concept of electric potential meaningful.