Voltage Indicator Part

I am building a test fixture to testing a device, a router of sorts. I have LEDs and switches for my outputs and inputs respectively. I also have a lot of RJ45 & RJ11 ports. I also have a handful of DB9 connectors. Off of my router I am testing I also have two power pins (+12v & +12v return). Does anyone know of a simple way to indicate 12v on those pins? I was thinking a simple LED but this wouldn't show any under or over voltages. Does anyone know of a simple voltage indicator display component that might work for me? Something simple but still able to show what voltages are on these two pins to at least a 10th of a volt but a 100th would be better. Thanks so much for any resources your can direct me to.

 

These ideas will work perfectly with my project. Thanks so much for the push in the right direction. I had no idea they made these little convenient modules.