Reading spark plug.

I've seen a few tests were additives didn't really do much...My small cam 340 will detonate on 91 and always feels like it's being held back even when you can't hear it. On 93 octane it runs perfect and pulls hard.We're lucky, it's available here at a few stations.
I have ridden in friends' cars that really roar and just sound a bit "off" and don't pull like they should. I tell them it's detonating but they don't believe me because they can't hear it. It's a few bucks more per tank that i gladly pay.