Trying to "psyche" your opponent out with the perceived threat of a power spray is just silly mind games. I don't see any harm in it. I've always found that it's easier to get a read on someone when they're actually doing or saying something, than when they're maintaining a calm expression and posture and keeping their hand to themselves.
Pay close attention to your opponent's turns. Keep a mental note of the cards that go into his hand (like the things he searches for with a search supporter or power), the things that leave his hand (stuff coming into play or going to the discard), and whatever is left remaining are your "unknown" cards. Probability and the plays your opponent makes will help you decide whether your opponent has the power spray, or the pluspower, or the warp point, or whatever card it is they need to beat your next play.
I love the concept of SOTG but I don't understand how the "bluffing" (actually really poor bluffing) that has been described in this thread detracts from the fun and sportsmanship of the game. I think bluffing is a fun and exciting dimension to any game where there are "unknowns" like face down cards or cards in hand. I always thought the "honesty" aspect of SOTG just meant not cheating. If someone wants to be "dishonest" about what's in their hand, I don't see how that's causing anyone harm. You don't have to believe them. You don't have to disbelieve them. You haven't actually -seen- their hand so to a decent player it doesn't matter what your opponent says. You're going to try and make the best possible play either way.
I'd much rather have my opponents try and tell me what they have (or don't really have) in their hand. It's so much easier to figure someone out that way. ;]