Use LEDs to indicate voltage
The device you need is called "dot bar graph driver", most commonly implemented as LM3914 integrated circuit. Here are instructions how to use it. You just need to adjust voltage dividers to suit your particular levels.



The LM3914 can safely operate up to 25V, so just connect the net labeled (5V) to positive terminal of your source, and ground net to negative terminal of the source you need to monitor. The circuit, however, will consume maybe 10-20 mA, so the source should be capable of supplying this current without affecting its voltage.