On battery operated circuits you simply need a simple way to test the voltage. In other hand complicated circuits don’t look very attractive. So for keeping things simple and low cost, Einar Abell suggests his single transistor voltage indicator circuit. It is able to detect transition between two voltage levels on battery.
Simply speaking, if circuit is powered from 9V battery, then it will transition from green to red indicator when voltage foes from 7.1V to 5.8V. One downside of this indicator is that it drains about 1mA of current. For any battery this is nearly not acceptable. To avoid constant drainage simply drop a push button to test voltage when you need.