Honda's 'main relay' is a combination relay, actually a couple relays together in one case. Provides electrical power for the ECU, fuel injectors, fuel pump, few other things. They're legendary for getting flaky when they're too hot. Located a few inched above the hood release lever, above your left knee somewhere. I suppose the worst time would be after the car sits for hours in the hot sun & it's 400deg. inside??

Pull a spark plug wire out, find any old spark plug, plug it into the wire. Lay the threaded metal part on a good engine ground like the lifting lug. Watch while someone cranks the starter. The spark should be strong & blue. Weak yellow spark is not too swift. No spark at all is, well, not good. You can use a screwdriver & hold it with a gap, but you might get a good shock. (You don't have a pacemaker or anything???)

When you first turn on the key ("ON" not "START") the fuel pump should run for 2 seconds then quit. It's not real loud, so turn off all the fans & stereos before you try. The pump is in the gas tank so you can hear it better at the fuel filler cap. This doesn't prove that your injectors are actually firing. There's more sophisticated tests to do next...

If you're not planning to repair it yourself, some mechanics don't like to take your word for it, even if you've checked out this stuff. But it's always good to have a coherent explanation about what's going on.