How can you know when someone or something exist?
I suppose a concrete object or observed phenom is said to exist if it has characteristics or qualities. Abstract on the other hand, up for grabs. For instance some of us pantheists see God as almost purely abstract entity and whether She exists or not as a purely semantic issue
Can you really prove someone consciousness?
In ordinary terms if he seems rational we suppose him conscious. But if you knock him out he’s supposed unconsciousness regardless of any activity that may still be taking place in his brain
How can you know that every human beings in this world have consciousness not only your self?
You can’t, except for common sense and intuition