I do think that we are overly capricious in assuming that we know what's going on in somebody else's mind. Really, a lot of times we're baffled by our own minds. That seems to be an important ethical consideration that people take for granted. I don't think that I can recall a system of ethics outside of Noam Chomsky which dismisses intent.
Still, I have to say that I do make a few of those assumptions myself based on indicators.
The documentary about James Randi was aptly named An Honest Liar. Randi made a career as a magician before switching to debunking psychics, mystics, and faith healers. James Randi followed the path of Houdini, who started debunking mediums after his mother died. Penn and Teller have followed in the footsteps of Randi. That's the weird thing about magicians... I'm starting to think that they're among the most trustable people on Earth. Even when they're lying to you, they're telling the truth about it.
Switch to people who are basically magicians but claim that what they're doing is for real: psychics, mystics, mediums, etc. I can't always tell you if people like John Edward, Silvia Browne, or Rosemary Altea are lying or if they actually came to believe their own bullshit. We know that the claimed "faith healer" Peter Popov was lying. We also know that people, for some reason, went on believing him after Randi kicked the figurative crap out of him. I do feel that Edward, Browne, and Altea are lying to us; but, I have to admit to not having any proof that would stand up in court.
Self-deception is a deceptively (pun intended) powerful thing.
I do think that we should assume good intentions until we become throughly aware of ill will. Still, John Edward probably is the biggest douche in the universe.