I would suggest trying one of the battery tender chargers and seeing how that works. I have several of them and they will all tell you the state of the charge of the battery when you connect them, and then show you the state of the charge (% of capacity) as they charge, and will then show you and battery capacity when the finish. They're fully automatic and you can connect them and then just leave them alone and let them do their job. Come back a day or so later and see what they report.
This is the one that I've used the most
https://www.amazon.com/Schumacher-SP1297-Automatic-Battery-Maintainer/dp/B0756PV96N One of the nice features of this model is that the outlet cord has a 2 pin plastic plug on it and the charger comes with two different ends for the cord. One cord has ring terminals so it can be semi-permanently attached to a battery and then the cord just plugged in when needed. (See picture #3.) Or the other end has the usual crocodile clips (picture #1). One of my cars (a MB) doesn't get driven very often and this model is notorious for running down batteries and then having serious issues, so I attached a cord to one of the two batteries in the car and the plug is located in the trunk. Now all I have to do is to open the trunk and plug in the maintainer and I can leave the car for as long as I want and not have to worry about what happens to the battery.
Once you have
fully charged the battery then you can do as RoGeorge suggested and just use a couple of automobile headlights to load test it. Connect up as many lamps as you need and let them run for say an hour and then disconnect the lamps and reconnect the battery tender and see what the battery state is. If it's lower than expected or if the battery fails to charge initially then you know that it's bad.
FYI I think that most of those powered wheel chairs run their batteries in series. If so then I don't think that you're going to find many where only
one battery is bad. My mother worked in her church's thrift store and they had many powered chairs given to them after the owner died. But I don't think that they ever got one that had any good batteries. Many had not been used much when the owner was alive (many were so disabled that they couldn't get in and out of the chair by themselves) and they also hadn't been recharged regularly so that would kill the batteries even when the owner was living. But also after the owner's died, the chairs would sit around for many months, and sometimes years, without being recharged (and usually the charger was lost) before the family or the estate donated the chair to a charity. So again the batteries were ruined.