How do you know if your battery is weak?

[skaler]

This 2002 Acura was purchased from a small used car dealership. I took it to an Acura dealer for a thorough inspection($100 charge).
Among other things, the inspector said the battery is weak and needs replacing. It has one of those plastic "eyes" and looks green at the moment. Did he put a load on the battery and noted it was failing, or did he just write something to justify the $100 fee?
As an aside, my experience was one of the ugliest in recent memory. I was accosted by a used car salesman, and he and the inspector did a song and dance to try and convince me to take the car back and come to them for a "certified" used car. I only look dumb. I'm still contemplating writing a letter to the general manager.