We truly can't, and the names we give it are all semantics. We use shadow people as a name for those things that tend to act the same, ghosts for others, spirits for others. We classify the energies per their actions or what we perceive of them. What we are really doing is assuming a lot on the bits of evidence we have. What we have to be very careful NOT to do is assume anything is benign. It CAN be, but to assume anything is, without some evidence, is a bit on the dangerous side, as many dark energies play nice at first. This does not mean we have to fear it, but it does mean we have to discern very carefully how we interact with any energy/being/entity.
To those of you out there that think it's nice to have an energy hanging out with you, let me remind you that this is NOT their realm. They are not supposed to be in your life. Unless the energy is a visiting Angel/Spirit that briefly comes to deliver a message and leaves, it is not supposed to be on this side of the veil. If they were, they'd be everywhere all the time interacting with everyone, and science, physics and religion all come together on this particular point.