I guess we can't proof to a real sceptic that something is real, because for most sceptics, seeing is believing, and if you don't see you don't believe. So until they really experience it for themselves, then it's harder to proof.
Not all of us are sensitive, but there are times, when the energy is really negative and you can't help but to suddenly feel scared and uncomfortable. Yes, it might just be the mind, but why is it that you feel this way in a certain place, and not the same way in other places, at the same time?
Learning Dharma confirms that there are negative entities everywhere for us because we learn about the lower realms. We learn that our negative karma leads us to the lower realm, we learn that attachment leads us to the lower realms. We have people affirm of the existence of other beings through their experiences, we have possessions evidently presented to us through youtube/friends/word of mouth etc.. What more do we want, for our minds to accept that they exist? The only thing stopping us from believing its existence is our own ignorance. Learning the Dharma, we strive to not be born in the lower realms, not to be a spirit. Therefore, if we don't believe because they are unseen to us, are we saying that we do not believe the teachings of the Buddha, or the teachings in the Lamrim? The lowest scope of the Lamrim, is to avoid ourselves from being born in the lower realms.. so it does mean that we are avoiding being spirits hence we cannot dispel their existence.