It is indeed a hard question that one could also apply to humans. There is no way to know if someone else is conscious. It might be obvious for all people, but what about babies or people with advanced stages with dementia or certain types of brain damage? What about people with mental disabilities? (I have experienced some of these in family members.)
Even for "normal" humans, the point is that consciousness is by definition unobservable from the outside, so everyone else but you can be un-conscious and you would have no way of telling. It's the p-zombies thought experiment.
