Well they can find out if you have put something on and taken it off. When they hook your car up to the computer then they can find out if you have changed the intake around beacause of some sensors. Now depending on the dealer it can not "void" your warranty if you put it on, but if it does cause damage to your car and the dealer can prove it then they will void the warranty for that specific part of the warranty and charge you what ever it costs to get it fixed. Ex. If you change your suspension out for springs and shocks and your car fawks up for some reason then it only voids your suspension warranty not the entire warranty.

-tom