A DMM is good for showing you the OCV of the batteries, and if you can juggle it, the voltage under load, but a hydrometer will give you a better indication of the actual condition of a flooded battery.
Good tip, thanks. The book I've been reading "Build Your Own EV" (appropriate to crack it open, now that we're half finished, eh?) sort of pooh-poohs hydrometer use in favour of a DMM, which I took at face value. But I realize in retrospect, they're expecting you to be using a nice set of shiny new batteries.
A brief note about electric motors (I apologise in advance if you already know this),