“How do you know?” is a really useful question.
People often assert things very confidently, without giving a reason. Sometimes it’s easy to respond to their confidence automatically, and believe them without thinking about it. Which can lead to a lot of mistakes.
The question “How do you know?” can be very helpful. Sometimes it’s something you can ask directly — some people are very receptive, and will think about their reasons and give you a good answer. Sometimes they know their reasons; sometimes they haven’t thought about it before and do think about it when you ask the question.
Some people don’t respond well to that kind of question, and asking isn’t always a good idea. But you can still ask yourself the question. Sometimes thinking to yourself “How do they know?” helps. Sometimes thinking to yourself “How do I know whether or not that’s true?” helps.
Therapist: My client is engaging in a lot of attention-seeking behavior.
You: How do you know she’s doing it for attention?
Or you to yourself: How does he know she’s doing it for attention?
Or you to yourself: How do I know if he’s reporting her motivations accurately?
Teacher: This shard of pottery shows a king walking a dog. We can learn from this that dogs were associated with high status.
You: How do you know that it’s a king in the picture?
Or you to yourself: How do we know whether crowns meant kings?
Or you to yourself: How do I know if the teacher is right about this being a picture of a king walking a dog?
tl;dr Sometimes it can be easy to believe someone because they sound confident. One way around this is to get into the habit of asking “How do you know” or asking yourself “How do they know?” or asking yourself “How do I know if they’re right?”