They're easy to make; as noted, LED, resistor.
However, there is a major easy-to-use cable tester I saw from the George L's cable people. It's a little box that takes a 9V battery, and on one end there are two brass rivets with an LED in the middle. Plastic molding makes the cable ends want to lie in grooves touching the brass rivets. It's just the battery, resistor and LED, but it's extremely well set up and easy to use. I think they sell them for a few bucks. This is one of those things that formed plastic was invented for. It's handy, does EXACTLY what's needed, and is compact and cheap. Testing a cable for shield continuity, tip continuity, and no shorts is literally 1-2 seconds and can be faster with practice.