One way to make power measurement for USB powered devices is to make your own USB cable to allow for current measurement via a multimeter, but if you think it’s just too much hassle, I’ve found and purchased a small test device called CHARGER Doctor that shows voltage and current on a 4 digit display, and sells for $5.40 on DealExtreme.
The device features a USB female out port, and a USB male input port, that you can place between your USB power adapter, and the cable to your device to alternatively display voltage (4 seconds) and current (9 seconds). It can be used for power consumption measurements, to test if your charger is working normally, or finding out the power adapter that will charge your phone the fastest. It’s said to support measurement between 3.5 to 7V and 0 to 3 A, with an accuracy of +/- 1%.
I’ve shot a very short video showing the CHARGER Doctor attached to my phone charger.
Reviews on DealExtreme are mostly positive saying the device is accurate, however some people said it does not support data, whereas others don’t experience this issue. I’ve connected it between my hub and my camera, and I had no problem browsing the files. However, I would only get proper voltage readings, with current always showing as 0.00A, even when importing videos to my computer. Doing the same test with a USB thumbdrive worked apparently fine showing 5.15V and 0.08 A.