Install an app like Battery Monitor Widget.
It will actually track the charge current into and out of the battery. It will also let you know if it thinks it is charging off of a USB port, or an AC charger.
It have a number of chargers. One on a night stand. One on my work PC. One on the handlebar clip on my motorcycle. One in my car and one in the wife's. Occasionally when on the motorcycle, everything would indicate that it was plugged into an AC charger, but Battery Monitor Widget would never measure any current. I found the USB connector was corroded and a new one fixed it right up.