alternator light

please help! a few days ago my battery/alternator light started coming on... i took it into the shop, and my battery and alternator tested out well within normal range, so i was basically just waiting for the day that my car wouldn't start. today, however, the light is no longer just flickering, but is staying on for long exteneded periods of time, but the car still starts up no problem. could this be a loose belt? where is the polyrib belt? how do i test it?