By divine revelation, what else?
He saith unto them, But whom say ye that I am?
And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. -Matthew 16:15-17
Now, if I were able to independently verify your experience, that would be empirical, wouldn't it?
You still have to empirically demonstrate that other observers exist, otherwise you're rooted in something other than empiricism.
So, if other observers do not exist (even though you can see, smell or hear them), everything is simply a figment of your imagination and the point is moot -- the universe is whatever YOU think it is (which has greater theological implications than the theory of evolution even approaches).
Basically, you either have to accept the reality of other observers, or conclude that reality is simply a figment of your imagination and that your time might be better spent doing something other than conjuring such a complicated fantasy.