My top 2:
1. Interview customers to discover the right segment and if they care enough about the problem you want to solve.
2. Use a landing page to validate the solution and marketing channels before you build it.
Ask them to pay for a crappy version of the product.
This is key. People will talk a big game and then make excuses why they can't pay. Make a version and sell it.
This. The best customers are the ones who have already hacked their own crappy product, probably in a spreadsheet or some chat group. As long as yours is less crappy, there's a market for it.
that makes me very optimistic, I've located many related WhatsApp groups and telegram, the only thing that concerns me is how to deal with copyright, people seem not to have any problem infringing copyright on WhatsApp and telegram sharing files.
How do you find customers to talk to? I have one user who gave me a good idea (happened by accident), I think it will work. But I am no expert in that domain and I don't know how to find more users to talk to, before writing any code
The domain likely has tradeshows, trade publications, bloggers/influencers, etc.