Battery light came on and I'm not sure it's actually the battery


So the battery warning light flickered on my Corolla yesterday and then disappeared, then came back this morning. The car still starts fine and the headlights seem normal so I'm assuming it's not the battery itself. A mate reckons it's the alternator but I've never touched one in my life and honestly the idea of poking around under the bonnet makes me nervous. I found a Helpful guide to repairing a car alternator at home that walks through the testing steps with a multimeter, which seems doable, but I'm worried about making things worse. Has anyone here actually tested their own alternator? What reading should I be looking for across the battery terminals with the engine running? And is it worth attempting or should I just book it in somewhere before I get stranded?