Keep in mind, you're not just watching from a single position. Range finders and binoculars all over the ship would be watching, from different angles. The bridge deck log and the logs from the various range finders would all be compared after the battle to verify the information submitted in the official AARAssuming that one splash wasn't directly behind another splash, that should work. Still I can see if this happens several times (missing splashes and no explosions), they would start to question their shells. Especially if multiple ships report similar issues.