From D'Addario:

Part of the D’Addario DIY Solderless Pedalboard Cable System, the D’Addario DIY Solderless Cable Tester makes it quick and easy to test cables for proper connections and shorts. The Cable Tester can be used for both 1/4-inch mono audio cables and 2.1mm power cables and includes a convenient reference guide on the back of the unit. CR2032 battery included.