HOW DO I KNOW IF I'M NONHUMAN?

You will know. There is many different ways to be nonhuman, or even half human. Whilst not all nonhumans feel discomfort, or simply just abnormal within a seemingly human body, it is often the first sign of being nonhuman. It is incredibly common to be unsure of what you truly are within the first discovery era of your identity, I myself have gone through several different identities before realization of the Awakening I had gone through of becoming vampiric. It is also possible half more than just one nonhuman identity, for example, whilst I am a vampiric being, I have also connected a shard of my soul to a doll, making me both a vampire, and a doll. You can also have multiple levels of identity, for example, being a vampire, whilst also feeling a strong connection to cats, perhaps you are not a cat entirely, but identify enough to them to consider them part of your Self.