These are what I used - https://www.amazon.co.uk/Laser-2780-Ignition-Spark-Tester/dp/B003AN1VPW/
They are a useful tool to have (better than my mates way of diagnosing a spark or lack of spark by holding the lead near the engine which frequently ends up zapping him and is so hit and miss to be nearly useless). They proved my suspicions that the coil pack on the Disco had started to fail (and more importantly which one had failed at the time, though the other one followed shortly afterwards) and can show up very slight faults that you can't necessarily see at idle which show up under load.
If you have 4, you can stick them on one side at a time and compare - I've used them about 3 or 4 times now (got to use them tomorrow actually as well), you can see a weak spark by a dull light or flickering thats different to its neighboring cylinder. You just put them onto the end of the lead and then onto the spark plug.
I don't know what the GEMS coil packs are like though, they fit the Thor ones easily, but some other vehicles with long spark plug wells may make them impossible to use.
But, you could just spend the money spend on them on a new coil pack and get an answer that way!.
Incidentally the last MOT the Disco had, ended up with replacing the whole exhaust due to failing emissions both CO and HC too high (the best of the local testers I use as the others I've tried I didn't want to go back to a second time, won't test on gas and I really can't be bothered to argue the point with him any further now) when tested on petrol, the cats on that had suffered misfires both from before I brought it (as a spares or repairs with misfiring) and the failed coil packs, to the point where the exhaust rattled when idling - a sound like a stone caught inside it is typical of a cat thats broken up internally, so I wasn't really surprised that it failed emissions after scraping through the previous year (when the same garage had a different tester on, who did test it on gas once I'd told him what to select off the test machine as he didn't seem to know if it was CNG or LPG he was dealing with).
It sailed through with a new Cat, took the opportunity to replace the rest of it as the middle box was more exhaust putty and rust than steel where it had been patched up before and hacked about where the flange on the end had rotted away. Only other thing I did to it was give it an oil change and a new air filter. If you wanted to check the cat, really the only option you've got is to stick a camera up it and look that way, or take it off and tip it up and see what falls out though that only tells you its broken up. They can get fouled up with other crap and stop working that way as well of course (oil being the most likely cause since you can't buy leaded petrol anymore)