A niche is a group of people with an identifiable difficulty. Profitability is tested through an offer, costs and sales; search volume or follower counts alone cannot establish it.
Narrow the subject
Move from a market to a problem: organization, then freelance organization, then follow-up tracking for independent workers. The last topic makes it easier to identify whom to talk to and what tool to offer. Still leave enough potential readers to distribute the product.
Collect evidence
Read recurring questions in communities, reviews of existing products and descriptions of paid offers. Record the date, link and wording. A single complaint is a lead; several independent conversations warrant closer research.
Compare the options
Consider four dimensions: evidence of the problem, an audience you can reach, demand for a solution and delivery costs. A comparison grid helps with decisions, but it does not turn an assumption into proof. Include support and production time.
Test a small offer
Show a table of contents, sample page or prototype to relevant people. Ask what they already use and where they get stuck. If you take preorders, specify the contents, delivery date and refund terms. Separate compliments from actual purchases.
Decide from the results
Keep the idea if feedback describes a specific need and you can deliver a credible solution. Otherwise, adjust the audience, problem or format. There is no universal number of messages, deadline or sales threshold that validates every niche.