"How can you be sure, if you are sure, that this movement was caused by PK as opposed to an observing spirit - the Ouija board idea?"
Nothing in the experience of PK warrants that sort of explanation. Why would an observing spirit require me to have a certain mental state (focused inattention)? Why would an observing spirit cause physiological sensations in my body, when the phenomenon is outside the body?
I would also tend to argue that Ouija is likely not spirit-related. It's an unconscious part of the personality manifesting through the ideomotor effect. The one thing that would debunk this is Ouija performed by a blindfolded person. I could never personally get Ouija to work, because I think I'm not a very suggestible person.