Wattmeters are also available for a little more from US vendors on ebay or amazon. .I have a coup[le of the latter type. They are good for telling max current and the AH used.

In my opinion, the working capacity of your battery in AH is a useful indicator of its health, If that value is close to the original rating, the battery is in good condition. If it's only half, well, you're getting half the range and you already know it's a sad battery, but a wattmeter confirms it.

Ride your bike til the battery is almost flat with a wattmeter hooked up, and look at the AH that were used. If you ride til it's really flat, the wattmeter loses its data. Or you can put it on the charger line and measure how many AH are needed to recharge it. You can also use a light bulb for a load, instead of riding. SOme users talk of using hair dryers. Whether discharging or charging, power is conserved. You should get the same number.

Again, if the battery shuits off, the wattmeters lose the data. Ive ridden on and on to discharge the battery only to lose the data. They do have connector for an external battery. A 9 V transistor radio (who remembers those) will power it. Then you can let the battery go flat.

The battery shops mentioned by the OP are probably only set up to do load tests on 12 and 24V batteries. Maybe they can do a 36.