Why Electric potential, or potential difference, is measured in volts, and the instrument that reads volts is a voltmeter. It is connected in parallel across the two points whose difference in potential you want to know, and it has very high internal resistance so it draws almost no current from the circuit.
Watch out A potentiometer sounds like the right word, but it is a variable resistor used to adjust a voltage, not an instrument that displays a reading. An ammeter measures current in amperes and goes in series; an ohmmeter measures resistance with the power off.
Potential = volts = voltmeter, connected across (parallel). Ammeter goes in line (series).