Which instrument would you use to measure electric potential or electromotive force?

Another term for electric potential or electromotive force is voltage. A volt is actually the unit we use to measure difference of electrical potential, by the way. To measure voltage, we use a voltmeter.

Another Answer

'Voltage' is another word for 'potential difference' (an electromotive force is simply a source's no-load potential difference), but it is NOT synonymous with 'potential'. In simple terms, 'potential' is equivalent to 'height', whereas 'potential difference' is equivalent to 'difference in height'. Potential is relative, because it depends on its point of reference (usually earth or ground in most practical circuits, but it can be any point elsewhere in the same circuit), whereas potential difference is absolute. Potential and potential difference are both measured in volts, and the instrument used to measure them is the voltmeter.