When you can independently verify it; otherwise, there is, as of yet, no perfectly accurate way to know whether someone is lying. Even the most sophisticated “lie detection” equipment (which no average person could afford) is not infallible (far from it). And so when you know your equipment isn’t infallible, you can never truly know when someone is lying.
https://www.quora.com/When-do-you-know-your-spouse-is-lying/answer/Eric-Johnson-311
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