How is a "Charged battery" on an Ipod touch indicated?

I just purchased an Apple Ipod Touch. Eager to begin using the device, I charged my pod on my computer while the computer was OFF. Smart! 😕 Well, realizing my error, I am now charging the device via electrical outlet. The charger's light is on, red 😠. And its been charging for more than four hours. How can you tell if the device is fully charge?