No not exactly. If you replace your screen or leave significant evidence of opening the phone they will see. When replacing the screen some people forget to remove the numbered stickers from the old ribbion cable and carefully place it on the replacement ones. This does not mean that it is 100% certain that a skilled technician will not be able to tell so use extreme caution if performing this proceedure. Be sure to look over ifixit's iPhone 3GS teardown before you do anything. Otherwise if you are opening the phone to look around be careful and have fun (do not forget to use caution). Hope that helps!
-Varsity