Well depends on how you frame the question right? To validate your idea you have to ask the right questions and penetrate all the bias and politeness.

So for example you can ask them this:

"Give me your honest answer. Let's say I had the product ready and finished and perfected right now... would you or would you not buy it from me?"

You really need to drive at this, you don't need to actually "sell" it. Validating an idea isn't a casual thing, but you don't have to go as far as actually selling it.

People will still say they'd buy it and then not actually buy. The only way to actually verify that they'd pay is to, well, make them pay for it. Collect their money first and then if it doesn't work out, offer a refund, like Kickstarter does.

Not true. You can frame it in a way to get the truth out. You gotta dig past the surface level bs.

It is totally possible to get these real answers out of people. Your human, they're human and are able to simulate hypotheticals quite accurately if the simulation is framed correctly.

In fact I find it highly highly mistaken to think that it is fundamentally impossible for humans to answer that question.