So a few months ago, I stumbled upon a thing at Adafruit.com called a "Charger Doctor" which was really just a Multimeter designed to slip inline with your USB cable. It was $7.50 and I liked it.

We may receive a commission on purchases made from links. One tool you'll find stashed in all the best electrician's tool backpacks is a multimeter. Most modern multimeters are of the digital variety, ...

Model 2100 is a high-precision, 6½-digit DMM with 10-Vdc accuracy of 38 ppm at a measurement rate of 50 readings/sec at full resolution, and over 2,000 readings/sec at 4½-digit resolution. Aug. 9, ...