I snooped a little as a kid/teen. Some of the boxes were pretty obvious when shaken (LEGOs for instance). And there were times when my parents would pull the tag off of something and throw it in the trash. They'd forget it was right there on top and ask me to take out the trash.
For the most part, now, there aren't any surprises on Christmas morning. I'm either with them when they buy it, or I tell them that's what I want. (Though snooping is pointless now, as I've moved out and gotten married.)