This thing tells you the voltage on your USB port, as well as the amount of power being drawn by the device:
I’d never seen something like this before, but I really like the idea. I’ve never used it, so I can’t vouch for the quality.
If on a lower budget see: http://dx.com/p/usb-av-usb-power-current-voltage-tester-translucent-blue-silver-235090
Not sure about the accuracy
This is funny.
https://www.ghielectronics.com/community/forum/topic?id=13169
(same device in that thread that RobvanSchelven pointed out here)
Huh, I should get my head checked.
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@ godefroi - I am happy to realize that i am not the only one 
I have one of these (I got the one from ProtaPow UK, but delivered by Amazon). So far I have been happy with it. It does not seem like it is really high quality, but it is still working after having been plugged and unplugged a hundred times.
Right now my Panda II (with Connect) is using ~220mA, which puts it at a little over 1W. Since it only reads in multiples of 10mA, it is best used for an indication of what is going on.
As an example, on a G120 the power usage will change considerably 10s to 100 mA from idle to full busy.