I watched a video from Andreas Speiss reviewing electronic loads, and identifying one in particular called EBD-USB+ from zketech.com and purchased through Amazon or Banggood or wherever. The software is a little hard to find, but it’s available through this link. It comes as a .rar file, which you might need to unpack with 7-zip or Winzip. You will also need the driver.
The resulting Windows-only software allows one to test two categories of things: batteries, by allowing a set amount of current to flow and recording both current and voltage vs. time, and power supplies, by allowing an increasing amount of current to flow and recording voltage vs. current. The Speiss video explains really well.
Here are a couple of results I got while trying to locate a power supply capable of furnishing 5V at 2.1A to power a Raspberry Pi 3B. None of the five wall-wart power supplies really made the grade.
