Hmm that's a really good question, I'm going to have a think about it before bed and see if a good night's sleep can help, but I'm not too optimistic I'll be able to write a particularly good article to answer that.
The brief thoughts off the top of my head are that the two options are either to spend money (which doesn't help the people in your question lacking money to spend on it), or to have someone(s) with enough experience in the broad area to be able to combine their gut feelings with some fairly light research.
So perhaps the best budget option would be to find someone you could hire for a couple of days of consultancy who can bring their gut feeling, though a) that's obviously not free and b) if you don't already have the right connection it can be hard to know whose gut to trust.
But it's an article I'd really like to either write or see written, so let me get back to you tomorrow, and if I can't think of enough useful guidance to make an article from it I'll ask some friends and colleagues and see if maybe someone else could.
p.s. Would be grateful of a thread upvote to help get more suggestions :)
edit: Am I right to infer from what you wrote that you're primarily thinking about market research in terms of figuring out if there is a potential product market fit, or also market research in a wider sense in terms of things like understanding competition, figuring out potential pricing, etc.?