Checking the alternator on a Hilux (like Tacoma)

Hi Group, I would be grateful for instructions on how to check the alternator charge rate on my 1998 2L-T engined Hilux (similar to a Tacoma). The twin 12v batteries won't hold a proper charge more than 2-3 days and I suspect they need replacing, as they look like the originals. The vehicle is still under guarantee and the dealer who sold me it says it may be the carbon brushes on the alternator (which would save him the cost of giving me new batteries, of course). Indeed it may, although I think the batteries have had it, as they go flat in 2-3 days without use of the vehicle. I would like to test the alternator before I take the vehicle to the dealer. I have a multimeter. Can anyone say where I stick the probes, and what the voltage range should be. And how to make sure I don't damage myself or anything else in the process? Thanks in advance, Michael Anders (1998 Hilux 2.4D XtraCab)