When I was three years old, I reached into a bag of potato chips up on the counter in the apartment my family lived in then and drew one of the chips out. Just then, my father walked into the room and caught me munching. "Who said you could have that chip?" he asked in his booming voice. I stood there stunned, looking at him like a deer in the headlights, a half eaten chip motionless in my frozen hand.
"Mommy," I said in response, eyes wide and voice trembling.
"Really?" my father said, disbelieving. "She said that from the shower?" He was right, my mother wasn't anywhere nearby -- she was in the bathroom getting washed. I could hear her singing from the kitchen. "Let's ask her."
I sat fearfully as my father walked into the bathroom and I heard him ask my mother if she said I could have a potato chip. I couldn't hear her response, but I guessed what it would be. My father walked back into the dining room. "She said she didn't," he said. I just stared at him. "You can't have these without asking first," he said, taking the bag away and putting it out of reach. I'd been caught red-handed (or salty-fingered). Fortunately, I escaped without a spanking, after I pleaded that it was "just one chip."
Ever since then, I've had a lifelong fascination with learning how to spot a liar, and how to not get found out oneself when on occasion pressed into lying. I hate lying, and avoid it whenever and wherever possible -- I wasn't very good at it when young, and came to the conclusion that lies always surface in the end, so it's better to just be honest -- but I'm all for being self-reliant, and very occasionally sometimes you've just got to know how to do it.
So how do you tell someone lying? There are a bunch of ways, and learning as many of them as you can is something that will benefit you enormously in all of your social interactions and relationships and friendships and parternships to come.