Best way to test/comparo equivalent rated power banks?

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https://www.ebay.com/itm/0-99999-mAh-Cur...3256658507

EDIT: I have a similar model and it works fine. No experience with that one, it was the first one that came up.

Here is more options:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1311.R1.TR5.TRC2.A0.H2.XUSB+current.TRS0&_nkw=usb+current+meter&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=USB+mAh+display