How do you know when your kids are lying?

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My daughter is just now getting into the lying stage. Not really bad lies, cause she's only three. Just the small ones. "No mommy, I didn't pick up the cat!" Stuff like that. Nothing malicious. And, there's no way to prove that she didn't. How do you know when your kids aren't telling the truth? Are there little cues or signals that they give off? How do you deal with your kinds when you know they're lying or being less than truthful? Time-outs? This is new territory for me.