[walrus01]

USB type A male-to-female inline ammeters are really cheap, and accurate to 0.1W. I got one for ten bucks.

[foobar1962]

How do you know THAT doesn’t have a surveillance device inside?

[walrus01]

Sort of a "it's turtles all the way down" type problem, but one could always x-ray it.

[metaphor]

+/-20mA seems a bit coarse...with modern low-power silicon, imagining it wouldn't be too difficult to skate under that radar.