i don't think your opponent should get to know which slot you used it on. They should know that you used it. *Guardian, not Gaurdian. Nice card!
I don't think there's a choice, as your opponent could see you use quanta and could see one of their creatures not attack (and well, the fact that you have this permanent out at all is probably enough evidence ).
Well, you are right, once the effect triggers on the creature's attack they will know and its a moot point... but I think what they were getting at is whether or not your opponent gets to see what slot got targeted BEFORE their creature attacks (or trys to use a skill).
For instance, suppose you have several growth type creatures out, but only enough quanta to power one of them. If you know which slot is targeted, you simply use the growth ability for the one in that slot. It won't get +2|+2, but it will still get its attack. On the other hand, if you can't see what slot is targeted you just have to guess and hope you pick the right one.