I have no clear memories of the Santa deception bar two:

We left a carrot out for the reindeer (along with a beer for Santa, which should have been a dead giveaway, really) and it was only half-eaten the next morning. Having been recently caught lying, I had figured out vaguely that the liar's reaction to false accusations is denial, and the truthful person's reaction is confusion. I wish I could say some spirit of rationality possessed me to check if my parents were lying about Santa, but it was merely me trying to catch my parents in a lie because they had scolded me for lying previously. I remember asking if they didn't like eating raw carrots, expecting denial or confusion, and being surprised when they smiled and owned up to the whole Santa lie.

The other was going to Religious Education at about age 6, hearing about God, applying my Santa-discovering skills, and being frustrated when the RE teacher refused to own up to the lie.