A bright idea!

I have been thinking that I should get a new battery for the bike on the next service, if not before, as I don't know how old it is. I suspect it's the original because it is a Harley Davidson branded battery and the bike is now coming up for 7 years old.
I normally keep it on the 'Optimate' battery charger/conditioner which charges it up and then checks the charge every so often just keeping it topped up. You can leave it on an Optimate charger continuously all winter if you want.
a couple of weeks ago I hadn't ridden it for a fortnight without the charger connected. I rolled it out to check it over and found it wouldn't start. Turned over a couple of times and then died so put it on charge for a few days until yesterday.
So, after yesterdays funeral, I wanted to leave it off the charger but check on any battery drain. This means taking the seat off, the side panel off and putting a voltmeter across the terminals, but then I had a light bulb moment.
I have a dual USB adaptor that fits into the 12v  'cigarette lighter' (who uses them as cigarette lighters any more?) but this one shows the battery voltage level. So I thought "Why not just put it into the socket every day just to keep a note of the voltage". It only takes a few seconds and it currently shows 12.5V. I think it's accurate as the socket is connected directly to the battery + and - terminals.
I'll see how it goes over the next week.
I'm hoping someone with more knowledge about such things will tell me if there is something else that affects it such as the CCA (Cold Cranking Amperage) for which there is no way of checking as far as I'm aware.