I don’t have a recommendation for a tester, but after about a decade of vaping and dealing with 18650s, I will say it can be beneficial to keep batteries paired together (on devices that require more than one cell) during their lifespan so that you don’t have one sagging from old age while the other is still performing well. This probably isn’t as big of an issue on normal electronics that don’t have a massive current load like an ecig, but something to keep in mind. I typically never break up a pair but when I used to have several pairs of batteries, I would just use a sharpie to mark a full set “A” or “1” while the next set is “B” or “2” to avoid mixing them up.

I use This from aliexpress. I got it for $1.99. reliable tests any kind of battery.

I’ve had a lot of success with this guy.

https://www.liito-kala.com/page92?product_id=6&brd=1

Supports a variety of different battery sizes, can charge at different rates, goves voltage readouts, and can run full capacity tests.

I was originally only looking for an 18650 charger, but I got way more than I bargained for.

If you want to just test voltage (and not capacity) a standard volt meter will suffice. If you are wanting to test rechargable batteries for their capacity then you will want to cycle them with recordable device that will count the AmpHour (or miliamphour) for discharge and then charge. For this I use an opus3100 tester. Or RC battery charger /discharger

any voltmeter will do, and then… you have a voltmeter!

if you want you can put it under load and test as well.

I’ve always used a multimeter and never had any problems with it