I just use 3 x 12v 50w MR16 lamps wired in series on my 36v li-ion battery to present a known load (some I know use older filament-type car headlamps instead). 150w is a fair/middling sort of load for a 250w e-bike motor system.
With a 36v e-bike battery charged to its' max 42v, and with a multimeter in parallel to monitor voltage, it's a simple case of timing how long it takes for that battery to drop to the minimum (the maker specifies 31v for my 36v battery). On my 10Ah battery, 150w/36v=4A load, so for me it should take roughly 2.5 hours.
At least measuring capacity in this way lets me know that my battery is (or is not) still delivering something near the makers full quoted capacity of 10Ah. Obviously if that total capacity starts to diminish by any notable degree, I know one or more cells are fading/failing and a new pack is likely on the cards. Otherwise, given all the variables of battery performance (bike/rider weight, use of power etc), it's just guessing as to whether your battery is still performing as it should.
This Bosch device doesn't appear to be doing much more than this either - present a load and measure over time.
Unless you can take out and test/balance each cell individually (or tap into each group of cells via the BMS), that's about all you can test, isn't it?
