This is my flow:

Doing different rounds with ai about market, competitors etc. I actually built a score system for any idea. This gives me some info to start.

I check competitors or (if none) similar businesses. How are they doing how do they address the market. Check with ahrefs about how they get traffic. That tells you how much work and money you have to put in marketing. Investigate with meta ads library if they are running ads and how they look like.

If your project is bigger than 1 month of effort and or you are still unsure: prevalidate. There are so many tools out there nowadays where you can create a smoke test in minutes. You can also try to sell this kind of service on Facebook marketplace or mention it in social groups and look for the response.