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Before you break out any tools, make sure you're actually seeing a charge problem. Check your dashboard - does the battery warning light stay on while driving, or are your headlights dimming noticeably? If the car starts fine and everything runs normally, you might not have an alternator issue at all. Sometimes it's just a loose belt or a corroded battery terminal causing trouble.
The multimeter test is solid (the earlier answer nailed that), but here's what folks often miss: you need to test under load. Turn on your headlights, AC, and maybe the rear defroster while the engine idles and you're checking voltage. A weak alternator might hit 13.5V with nothing running but drop below 12.5V the moment you load it up. That's your smoking gun. If voltage stays steady and climbs a bit as you rev, your alternator's probably fine.
One more thing - if you've got a volt meter on the battery and the voltage is rising over time while idling (like it goes from 12.8 to 13.2 to 13.6), that's actually a good sign. It means the alternator is actively charging. If it just sits flat, that's when you know something's wrong. A local shop can do a proper load test on the alternator itself if you want to be 100% certain before replacing it.
Get a multimeter and check the voltage at the battery while the engine is running - if it's above 13.5V, your alternator is working fine, but if it's at or below the resting battery voltage (around 12.6V), the alternator probably isn't charging. You can also disconnect the negative battery terminal while the engine is running and see if the car stalls; if it does, that's another sign the alternator is dead since the engine would be running purely on battery power. Before replacing anything, make sure your battery terminals are clean and tight, since corrosion or loose connections can fool you into thinking the alternator's bad.
The voltage check while running is definitely your baseline, but I'd actually start by listening - a dying alternator often makes a whining noise that gets louder as you rev the engine, and I caught mine that way before the battery light even came on. If you do grab a multimeter, watch for voltage that climbs above 13.5V when you first start it but then drops back down or fluctuates, because that's usually a sign the internal regulator is failing rather than a clean dead alternator.
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