EMF can be measured in voltage (potential difference) and is what moves charge around a circuit. Potential difference, on the other hand, is just a property of the charge distribution of the circuit.
You can measure EMF only when the source is not connected to a circuit, otherwise you will get a voltage different from the EMF.
For example, when you take a 1.5V battery and measure the voltage at the terminals with the battery disconnected, you may find 1.64V. This can be interpreted as how much energy can be used to push these electrons. In the case of an open circuit, 1.64V is not enough to overcome resistance and create a circuit through air.
On the other extreme, if you short circuited (carefully) your circuit and measured at the terminals you will find 0V. Charge would just be moving at large amps.
Due to the internal resistance of the source, you will never be able to measure the EMF unless the source is not connected to a circuit.