How to check batteries without a phone?

Question

Iron

I am engaged in trade, now I switched to batteries for smartphones. I don’t know how to check them, I used to order only those batteries to check which I can get a phone from someone, but now I want to order many others, but I don’t know how to check them for performance.
But there are still many questions:
1) I googled that the capacity (mah) can be checked using a Chinese device, to check it is necessary to discharge them, and then charge them through the device, but how can I discharge them without a phone?
2) After checking the voltage with a multimeter, can I assume that the battery is working?
Tell me who checks