The cheapest way to test an alternator is by using a voltmeter or multimeter. With all the car's accessories, lights, etc., turned off, have someone rev the engine to about 2000 rpm. With the voltmeter or multimeter connected across the battery's terminals the reading should be in the range 13.5 to 14.4 volts (i.e., a couple of volts more than the battery's rated voltage). Less than this and the alternator may be due for replacement. If the measured voltage is less than 12 volts then the alternator has failed.
Some alternator testers consist of an ammeter (a current-measuring meter) calibrated so that they can show the starting current and also the charging rate of the battery.
A more elaborate device is an alternator tester that can diagnose the charging system without having to use a meter. The tester isolates the alternator from the rest of the circuitry, making it is easy to determine if the problem lies with the alternator itself or elsewhere in the system.
At the other end of the scale, there are expensive systems, suitable
for professional shop use, that will test alternators, generators,
and batteries for full output. These typically include a heavy-duty
carbon pile for completely adjustable loads up to several hundred