Detonation/ preignition shows up on the sparkplugs as melted electrodes/ and or silver flecks on the porcelain. And I have seen holes hammered thru the tops of the pistons right in the center, and melted top ring lands as if cut by a blow-torch.
Brown tops is perhaps lean; if they smell like Dobeman DooDoo then lean for sure. This is probablyOk for cruising with a good cooling system, as long as the power circuits are richer.