If I know nothing about it, I'll say so. That said, if it's a consumer-facing idea, I likely
have thought about it, at some point, often before some of the people I'm talking to were even in high school. Many ideas keep coming around every few years, and people without any sense of history at all may be doomed to repeat it (instead of potentially avoiding the mistakes of previous efforts in the same area).
There's plenty of things I've missed, and I'm not at all any prescient genius at this, but I've been approached by people with 'great ideas' who 'just need someone to build it for them'.
One example from last fall - I liked the idea, but told them they were going to have lots of legal hurdles with the IP and content, and they assured me they wouldn't. Three months later, they told me they were having legal issues with the IP and content. They could have pressed on, but chose not to, because they wanted to finish up school instead. I think they also realized why this 'great idea' isn't being done - it's not because no one thought about it, or that it was some great insight, it's that it's hard to do because much of the content they want is tied up in legal issues.
But... wow - how about that? I looked at the problem for 5 minutes and saw issues they hadn't seen in 12 months of thinking about the problem. Why is that? Maybe because I've been working professionally for 20 years, and they were still in college? Certainly not claiming I'm smarter or more clever, but simply that sometimes having more experience in some areas lets you see issues that the person hooked on the idea hasn't seen (or doesn't want to see).
Your point about questioning is fine, to a point (no pun). Asking too many leading questions can make you come across like a jerk too.