I have never seen Satan or any spirit of any kind that I know of. (One can never be perfectly sure of such a claim.)
But, for a while there, I was Satan.
I know heaven simply due to the fact that I have experienced its opposite. For a while there, I was one of Satan’s demons living in hell. I was there, in hell. Hell is the complete absence of God or good. Hell is the complete absence of friendship and love. In hell, you are totally miserable, totally alone, and totally unloved. The pain is excruciating -- not just physical pain but psychological or spiritual pain as well. All you want is to cease to exist. You want it all to end.
I know heaven simply by the fact that I know and have experienced what heaven is not. I have gone to hell and come back to tell about it.
I hope and pray to God that I never have to go back there again!