If you use non-rechargeables, the depleted batteries are still good enough for other devices like alarm clocks, remote controls (tv, aircond, etc.), and even LED-torchlights. But then, you will have a lot of these batteries, and you can MOC like this:
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That is why I opt for re-chargeables, but due to lower voltage, I need to put in 7-8x instead of 6x. Hence, my MOC on battery pack couple of months ago.
Now, I want to use 9V DC adapter, since some of the stuff don't run far, like the excavators.
test-meter can get anywhere, normally I goto Pasar Road, Pudu in KL. Branded one is like Fluke. Yes, those with two cables, normally black and red.
Test meter quite useful, can test many things. AC or DC voltage. from 0.xxx to 1000V, even for cheapo ones.
Can test Amp, Ohm resistance, simple broken cable test, etc. Can use for home electrical also (don't simply plug into the 240V AC if don't know what you want to test

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When motor run slow, I just recharge the whole 6-8x lots of batteries. Quite conclusive on low-batt most of the time.
Very rare I check it often. But sometimes useful to isolate out a single faulty/flat battery, as it can cause the entire battery pack problem.

For general usage around the house and for your toys don't need to get Fluke, the price will make you faint! haha!
Lately I tried a very cheap brand "UNI-T", a digital multimeter cost me RM52.
So far so good...
Yeah, don't go poking those probes into 240V AC if you don't know what you're doing!
However, battery voltage is only a general indicator of the battery's state of charge. Voltage cannot tell you the battery's "health status", ie. its ability to store the charge. For that the simple way is to get a smart charger and analyzer, I've been using this brand for many years and I am led to believe it is one of the best around...
This is sold by Shashinki, a local online seller of photographic gears. Very established and reliable, DSLR kaki will know of them.