What does any observer have to evaluate the substance that presents itself to reason and senses? Only a single mind that resides within the observer himself. That makes for messy science.
Oh. Wait. There are other observers who cropped up by nature, all unintelligiently undesigned, who might have something to offer as corroboration. Listen to them!
Maybe you can;t because you have no method. However of 6.7 billion people on earth most agree their are no two exactly the same. However a few deny change and think themselves clones.