We can be pretty sure that Pluto (or other significant bits of the Solar System) isn't made of antimatter because we would see the affect of meteors and the like hitting it, for one reason. Comet Shumaker-Levy 9 produced an impact of the order ot 6,000,000MT when it hit Jupiter in 1994, though pure kinteic energy. You needed a small telescope to see that. Had it been composed of antimatter, the energy released would have been somewhat larger, about 4x10^10MT, or about the same energy as the entire sun emits in 1 second. I doubt a telescope would be necessary to see that, or possibly even eyes! Smaller things must hit Pluto often enough for someone to have noticed.