Each apple computer is registered within apples system with the exact specs that it has when shipped. If you change any of these we know right away. Also after seeing and opening thousands upon thousands of these we know what its like to crack open a fresh one and what it is like to crack open one that someones had their hands in already. There really is nothing you can do. That being said Apple doesnt usually get too upset about upgraded RAM or HDD. At a certain point they usually just assume that a licensed third party has installed them.
DO NOT send it to apple that way though. They will replace any parts they find to factory condition before sending it back as an insurance for their work. They will toss any upgraded HDD or RAM. It isnt pretty i have had people really angry for that but Apple has to send it back as the system was specced out of the box.