Ohm checks with coils can help but the problem is probably not that the coil is completely dead but weak. The best way to test a coil would be to connect an ignition wire to the coil, a good spark plug with the proper gap to the wire and then pulse the coil like the ignition module would and visually observe the spark at the plug *unless you have a high voltage probe; I don't know what the primary voltage is to the coils; perhaps someone else could say.
From what I understand on this year model is that a misfire is causing the Code 18 to set even though Code 18 is for something else. This year does not have a misfire indication for the computer but a misfire can trip the Code 18. If you have put in good wires and plugs then I would suspect the coils as the source of the code 18. I had even replaced the Ignition Control Module on my car.
If this was my car, I would go to Ebay and find the cheapest set of coils (check the shipping fees as some will be cheap but high shipping) and replace them. That is what I did because I didn't have the money and needed to have a car; kind of hard to walk 18 miles to work. It may not be the coils, could be something else but the mechanic in Ca told me he had a 93 Buick with Code 18 he traced it to a bad coil and it was the same for my car. So that is two different ones that had Code 18 on a 93 Buick due to a bad coil.
Yes, have it dark as it may not be a strong arch, you may just see some stray voltage. When I got the Code 19 a few weeks after fixing the Code 18; which was not due to what the official GM shop manual stated, I looked at the engine in the dark and noticed that the number one wire had stray voltage that would travel the length of the wire, you could see it. No arcing, just sort of crawling along the wire. In the daytime it was not possible to see it. The wires I had on the car were only two years old but I had brought a cheap pair due to no money woes. I got a mid-grade set of wires and replaced them and the Code 19 went away and the car ran fine.