There are a number of ‘methods’ used to check for HT cable integrity… some are definately dangerous (any which involve sparks in the engine compartment or personal exposure to the electrical energy from the coil). To avoid risks of injury from exposure to high voltage sparks from inappropriate checking methods, it is worth noting that standard automotive HT lead testers can be used on ULPower engines! These are available from a wide range of sources, generally for under £20/$20/€20 for a set of 4.
For testing purposes, simply fit them between the HT spark plug cap and the spark plug. Once installed, you can stand back and film the engine running (or not running). This method of testing makes spotting a failed HT lead/Connection/coil much easier and safer – and the videos can be rather fun to watch too.
Cameras have a frame rate and if you adjust your rpm to close to the frame rate, you may get a strobe effect allowing you to ‘slow down’ the spark rate – you might even spot firing sequences if you are observant!
WARNING: Do not forget that the propeller is spinning whilst filming (ideally set the camera up on a tripod at a safe distance). These testers are for ground use/testing only. Remove the testers, secure the spark plug caps and check them before flying.