Doing a routine maintenance on my tools, I have completely forgotten how I did this one. This is a tester I made when I was doing CCTV at one time. It tests for the correct polarity from the power supply and also if there is any shorting on the video cable.
For example, when I want to test if there is any voltage from the power supply, I would just plug it in. If the LED lights up, then there is voltage. But if the lit LED is red, this means the voltage polarity is wrong and you'd have to rewire it until the LED is green. And the for the video cable, if the LED lights up, this means there is a short in the cable. Simple isn't it?