Most people don’t like to lie. It makes them uncomfortable because they’re afraid they’re going to get caught. So they choose partial lies. People who use vague language select words like only, just, or a few. The word “once” is usually a red flag for me as well.
When a person is lying, they will often tell you that they don’t know. A truthful person who genuinely doesn’t know will say so but will chase it with an offer to think back or find out, because they want to reassure you with truthfulness.
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There are so many ways that I use to tell when people are lying to me or to their spouse. Way too many to write about. But probably the most reliable indicator is that feeling I have in my gut. It’s developed over time, and it’s reliable.
Perhaps your own intuition is as well.