- See which is the fastest power source
- Check if power supplied meets claims
- See how compatible chargers and devices are
This should be relatively easy to do with a multimeter, but I wasn't comfortable messing with USB cables. I couldn't find any ready-made devices to do this (unlike, say, for measuring power from wall sockets).
Well, I found the PortPilot, a crowdfunded project on Indiegogo. It's got male and female USB ports, so you connect it inline with your USB cable, and it shows you how much current is flowing and what kind of charger you have. The pro version even measures exact voltage and current.
I ordered the pro version immediately, and I look forward to reviewing it (and all my USB power sources).