How do you validate a business idea?

I did by finding my core audience in facebook groups, then posting a poll (asked moderators first).

YC's Kevin Hale has a good lecture about how to evaluate a startup idea, which is highly related to validating the idea. This question is a frequent one on IH and I mentioned in the past that I created a helpful tool (below) that gives you a score for your idea (you need to be honest with yourself and compare between your score and your co-founder's if relevant). I'm posting it again because it was super helpful for many entrepreneurs who already evaluated their idea: https://www.polidoesntcare.com/blog/the-startup-idea-evaluation-meter.

With Moon I validated the idea by building a "minimum loveable product" and released it as a beta as quickly as I could. To sum it up, the process is:
0. Confirm there is a viable market for your idea. (I found multiple threads on IH asking if there was an IH mobile client, and heard @csallen mention more than once on the podcast that people want an IH mobile app.)

1. Build the smallest version of your product that still provides some value. (I built an iOS app that can only read and filter posts but it does it faster and smoother than the mobile web version)
2. Get it into the hands of your target market. (I posted it on IH and Twitter)
3. Collect feedback and iterate until you have product-market fit. (In one month I've collected 50+ feature requests from TestFlight, email and Twitter)

IH forum posts that I find along the way:

• How to validate an idea

Focus groups

Let your potential customers validate the idea for you.

Get off your computer and talk to real people!

-Steveo

Admittedly a lot of what I work on is "problems I am directly aware of".

That being said, I found the book "Who Moved My Cheese" extremely valuable in thinking of things in terms of problem->solution->action. So, I generally think of things where the cheese has move, will move, or is too far away from the mice. I then think of a solution, whether I have the ability implement or not. Finally, I think of my own actions (research further, ask potential users, etc) and the actions others would take (would they pay for it).

I read something recently, but can't remember where, that discussed an experiment where people were asked to paint fruit and the art pros judged the outcome: https://www.opencolleges.edu.au/informed/features/why-problem-finders-are-more-creative-than-problem-solvers/
It turns out those who took more time to consider what the fruit looked like painted better pictures. The participants were tracked down many years later and those who were "problem finders" were significantly more successful.

I like this article by Lenny Rachitsky. It has a framework that forces one to answer some important questions before diving in.

https://www.lennyrachitsky.com/p/a-three-step-framework-for-solving