testing alternator

Voltmeter. Otherwise headlights on. If from idle they brighten as revs increase, then the alternator is charging. Maybe not enough - but it's charging. But a good battery shop should be able to test the battery. Mind you, they may say the battery is faulty because of a bad alternator, and whilst they replace those, they'll upgrade the faulty wiring and air-bags &.... But a $10 multimeter is a great investment!

To test your alternator, you need a DC Current clamp meter and a battery load tester.  Place the clampmeter on the wire between the alternator and the battery, connect the load tester to the battery and turn it on for about 15 seconds, with the engine running and someone keeping the revs up at about 2000 RPM, notice the current reading on the clampmeter.  That will be the amount of current that the alternator is putting out to try and keep the voltage up. 

Though that can be done, it's usually sufficient just monitoring the voltage. As Idiot Esq. wrote ..current that the alternator is putting out to try and keep the voltage up. Hence with reasonable RPM, headlights on, brakes, etc, is the battery voltage ~13.8V or more? But if you can check your battery voltage - unloaded (disconnect its -ve ground end) after surface charge removed (ie, lights on for a few minuted if it's been charging recently). A Full battery should be 12.6-12.8V. A flattery is ~11.3-11.6V. Fully charged but under 12.6V suggests a collapsed cell. But although 12.6V indicates a fully charge battery, it does not mean full rated capacity. EG - mine are 8 years old and still read 12.7V, but they only have ~10% or their original capacity.

Or plan C fully charged, should read as above, around 12.6VDC. Start engine, with engine running, petrol should be about 13.8, diesel 13.4. If lower or higher, e.g. 14.5 or above, change your alternator before it destroys your light bulbs, battery etc. Now disconnect your coil or distributor feeds or glow plug lead if diesel. Crank engine for 10secs. If the voltage drops below 10VDC, your battery is knackered.

How dare you imply my batteries are knackered! They are fine - unless I run out of petrol.... (My cold car cranks and starts at ~5.2V. I haven't tested any lower. Yet. And "cold" means somewhere upwards from ~50F/10C. But diesels lower than petrols? Charging voltage is based on the battery which at room temps means typically 13.8V - 14.4V where 13.8V is the float voltage (charge voltage once the battery is fully charged) and 14.4V the maximum long-term voltage for a 12V battery for acceptable gassing etc. (These are based on 2.3 & 2.4V/cell.) Fully charged is ~2.1V/cell which is ~12.6V. Above ~12.8V means either surface charge in the battery else the alternator is supplying power (though barely if ~13V). However, if petrol alternators are used without gearing changes (if diesels use lower RPM).... Otherwise I'd expect diesels to have a higher output voltage than petrols (having less load).