Its all part of the presentation and psychology. I don't really ask "did u see that" but if for some reason I have them sandwich a card between their hands, then I will not ask, but tell them you might start to feel something, perform the "magical move" bring tension THEN ask "do you feel anything"
Something else I learned from Wayne Houchin at one of his lectures . . . Wayne was performing his version of spoon bending, his version calls for the spoon to be placed in a paper bag, closed up then Wayne asks the spectator to hold the bag from the top with one hand and have your other hand underneath the bag. He then goes on to say " I want you to IMAGINE the bag you are holding is getting warmer . . .as you IMAGINE the bag getting warmerstart to pull your hand away from under the bag . . ."
By doing that, wayne pretty much told the spec that he won't really feel it but he has to imagine it, by doing that the spec responds positively by moving his had when the metal "starts to get hot" and that is what the audience will believe. Sorry for the long post, hope it helps.