Unfortunately its not quite as simple as that (wish it was). Really, you want a stable fixed resistive load and then measure voltage and current over time to give you the power output. Unfortunately the battery outputs are electronically switched which makes that very difficult to do. I would give it a go, but the cost of the batteries and risk of damaging them puts me off somewhat!Just had a look at my batteries. They are the older external BT e-8010 style.
Battery #1 - came with a 2018 Meta Power frame purchased Feb 2021 (i.e. a very old battery!)
41 cycles
97% health
Battery #2 - purchased after market, June 2021
26 cycles
88% health!!
Maybe they switched to some cheaper cells after a while and hence removed all the guarantees about capacity??
As for proving the capacity / range drop, I guess the best way would be to attach a mutlimeter to the outputs and sum up the current being delivered over time? Do multimeters do that??
I've got a cheap usb volt/current meter from ebay that does that for usb chargers.