I got a "Charger Doctor" from Adafruit. Not a very enlightening name but a useful device.
It alternates displaying voltage and amps (a few seconds each). The 7-segment display can make a capital 'A' (main image) but can only manage a lame 'u' for 'V' (insert).
Place your 'Doctor' between the USB source and your device to be measured. The image above shows the usual 5v in plus that my Raspberry Pi B+ with USB wifi dongle uses .32 amps.