4-20ma transmitters come in two varities, 2 wire and 4 wire. 2 wire is the most common.
If 2 wire then the device requires a power source (usually 24vdc through a 250 ohm resistor) to power the sensor so that it can then regulate the loop current according to it's sensor measurement. You can measure the value by either reading the voltage drop across the 250 ohm resistor (0% = 1vdc, 100% = 5vdc) or by measuring the loop current with a milliamp meter by wiring it in series with the loop wiring.
If the device is a active 4 wire transmitter then one can read the measurement current by just hooking up a milliamp meter across the the two loop output terminals. The other 2 terminals are wired to a DC or AC power source as specified by the specific transmitter.
Lefty