At work we get boxes of personal effects come through with signed decelerations stating what is in each box. Because we have to x ray each box we can tell (or have a very good idea if you are lying) At this point if the contents are completely different from what is stated that is when customs will start to sniff around the boxes and more than likely open them up and pull the contents apart to check why you felt the need to lie.

Honesty is the best policy, unless you really are trying to get away with something