That's one way to do it. Find a USB thumbdrive with a light that's always on when it's plugged in (and powered, obviously).
On a related note, I don't know the exact logic behind it but if I have my Kindle Paperwhite plugged into one of the USB ports on my iMac and a thumbdrive in another, I can power down the computer completely (thumbdrive light goes off) but the Mac will still charge the Kindle.
An Air probably doesn't do that because it's powered by a battery, but I figured it was good to know in case you test it with a device like a Kindle (with a charging light).