It turns out my problem was a physical short in the CAS signal wire to the boomslang harness connector. Quick jostling (like hard suspension hitting bumps on the freeway) would cause it to wiggle, and ambient heat would cause the insulation around the wire to get physically softer and not provide as much support to the bad connection. My folly was that when checking for bad connections, I didn't yank and wiggle the cable as hard as I should have. I was barking up the wrong tree the entire time, lol.

Is there any way to tell just by looking at them?

I just picked up this miata
https://www.miataturbo.net/cars-sale-trade-6/1991-turbo-mazda-miata-%245000-66076/
The ad states 660cc injector