How can I measure alternating current?
Use an AC Clamp Meter.
To measure any current, DC or AC, you need an Ampere meter, also called ammeter.
Use a DC ammeter to mesure DC and an AC ammeter to mesure AC.
Any ammeter must be connected in series with the circuit to be measured. In some cases this becomes complicated, because you have to open the circuit and insert the ammeter.
There is a way to measure current without opening the circuit, if you use a Clamp Meter.
To measure current with this instrument, all you have to do is to clamp it around the wire to be measured, without opening the circuit. Be careful to avoid electric shocks or short circuits, once the circuit will be energized (see figure below).
DC clamp meters are very expensive, but the AC ones are quite cheap.