When the viewer is connected to a voice server, the visualizer (white dot) appears above the Avatar. But there really is no visual reminder that the user has the mic turned on if the mic toggle is not in a PTT mode (I can only assume the SL viewer has a Toggle/PTT option, I know Firestorm does).
I think it may be helpful if the visualizer appeared in a different color, such as bright green, when the mic is on, and white or bright gray when the voice service is connected and the mic is not on.