gnu says "can't tell" if the generator battery is 6V or 12V.

A lead-acid cell is "about 2 volts" so a 6 volt battery has 3 "cells" in series inside the box and a 12 volt battery has 6 cells. It's rare to see a battery that doesn't have a cap, or on a very few of them a "view window" for checking the amount of water in each cell, one cap/window for each cell. On 12V batteries it's common to find two rectangular "stoppers" with three "plugs" on each one, so that when you pull each of the two rectangular blocks off it shows you three holes. You can nearly always tell how many cells are in one, and once you count them you know whether it's 6V or 12V.

You can also just use a voltmeter, since a battery is "effectively dead" at about 70 - 80% of its nominal voltage, and will still tell you what it is.

About the only exception is for the specialized "leakproof" batteries used in electric contraptions for the handicapped, such as "mobility scooters" or "electric wheelchairs," where the batteries are completely sealed and have no holes to look at. Very few of these use the normal "bolt" or "clamp" connections for the cables, and most of them have only a couple of little "tin ears" sticking out of either the top or the side. Batteries of this "odd" type are almost exclusively used with two in series and requiring a 24V charger.

Even the supposedly "spillproof" motorcycle batteries, which sometimes are 6V, still have the caps on them, unless they've been "specially equipped" for competitive motcross competition where sometimes the rules require the true "leakproof" ones.

Of course the safer method would be to go to Honda Power Equipment Owner's Manuals where it says you can download (pdf) a free manual, or order (for a price) a printed paper copy. The down side of this is
you do have to RTFM
once you get it. (Note that you may have to give them the the model number for your generator, and for some models you may also need a serial in order to get the right downloads. At the Honda site they'll show you where to look to find both if you need them both. They also offer "shop manuals" that probably have a lot more info, but I didn't look at whether those are also free downloads.)

John