As insignificant as they may seem, do not let their humble nature fool you -- cables are the most important part of any audio set-up. Without cables, there would be no way to transfer the current, and therefore no sound being played through speakers or recorded to disk. Because they are so critical to the success of your set-up, it makes sense to periodically test all your XLR cables, considering that they are likely the most often used, and to always check them before use. You can make a simple tester for this purpose, saving you a trip to the store and more than a few dollars.
Gather the components you will need and plan how they will fit within your project box. Because the boxes are of various sizes, you will need to formulate your own layout, but everything should fit, even in a reasonably small box.
Drill holes large enough to accommodate the XLR jacks, LEDs, and the switch.
Install the components listed in the last step in their newly drilled holes.
Assemble the circuit. For this design, the wiring is simple. Connect a wire to each of the leads coming off the XLR jack. Wire all these cables to the positive terminal of the 9V battery.
Run a cable from the negative terminal of the 9V battery. Split this wire and attach one side to the live terminal of the XLR jack. On the other side, install the switch. This will toggle between the ground and ring leads to test these components. For the installation, this means that two wires will run from the two terminals of the switch and connect to the remaining two leads on the XLR jack.
Install the resistor and LED lights before the leads of one of the XLR jacks -- it doesn't matter which you use, so long as the resistors and LEDs are attached to the same jack. Install the resistors closer to the battery and attach the LEDs closer to the jack. These are the LEDs that will light up to let you know if your cable is working.
Plug a cable into your new tester. If the cable is working properly, you will see the light for the live terminal light up. Upon flipping the switch, you should see the remaining two lights come on, indicating that the ring and ground leads are also functioning. If you see these lights come on before you flip the switch, this means that the ring is shorting to ground, which means the cable is not working properly. If you see no lights, you have a bad cable as well.
Wiggle the cable after connecting it to make sure that movement does not short the connectors.
Use a heatsink clamp when connecting the LEDs -- they're sensitive.