It depends, I think, on whether you mean physically see or hear, and therefore know is real or perceptually see or hear and therefore believe is real. Some people hear voices and see things, but these are sometimes generated from the brain and suggest some type of mental imbalance which often medication can balance again. I would imagine that other's reactions when a person asks 'can you hear those voices' is the only way of checking it out.
Then there's the medium who 'hears' voices and can often pass on important messages from 'the other side'. Do they hear them for real? I don't know.
We all see the world through our own coloured spectacles, tinted by our experience in life. A vet sees a dog as a patient, a cat owner sees the same dog as a threat to their cat, an allergic person sees the dog as an irritant, the artist sees a subject, etc. etc. etc.
Even if we all stood in a circle and looked at something, we would all get own view and therefore see something different.
But really, most of us see and hear the same things - people who are colour-blind see different colours to most of us and it becomes apparent as they label colours differently to most others - green and red being very similar. But do we see exactly what others see? I don't think we will ever know the answer to that.
Interestingly, my daughter asks 'how do they make red paint?' for red is a primary colour and "you can't make primary colours" - another of the world's conundrums.
I hope this has helped you to know the answer to your question. Feel free to elaborate if I've missed the mark.