Do your homework (both primary and secondary research). You have nothing to lose by spending a few days analyzing the opportunity.
- For a project that had to do with shipping labels, used UPS's , Fedex's Annual reports to estimate the potential number of customers who would prefer that kind of shipping. So looking at annual reports of listed companies that are in the same (or similar) business would be useful.
- Looked at platforms like Fiverr, Upwork etc. and scrape their job boards and analyzed for words that would describe what you are gonna build. Reach out to those customers who listed those jobs and freelancers who did similar jobs. you will learn a st ton.
For a project involving selling via messengers used - Facebook's and WhatsApp announcements page on their blog. - Popular Newspaper articles about digital advertising (that quoted sources) Sometimes, you can google things and when you find the news articles, they might indicate the source. For example "John Doe, an analyst with Analysis Paralysis Inc, estimates there are 3433 cows who eat ketchup in the West Virginia" . You can then google that company and find useful stuff on their blog or even buy the report from them.
- For a project that involved selling fasteners to fix ceiling panels, estimated the number required by looking at US Census Bureau data on housing square footage.
- You can also use google surveys to ask questions to the target market (the used to allow you to do free surveys). Yes they might ask for a faster horse. But you will get the insight that "speed is a need". So if you do a good job asking the right questions and interpreting the answers, you will get a fair idea of the opportunity.
Maybe you could post here what exactly you wanna know, there would be folks here who would know the answer.
Don't worry about folks "stealing" your idea. If you are worried about it, go spend 2 dollars on a lottery ticket. The chances are more that you would win the lottery.
Use tools like these to find EVIDENCE that people are trying to find a solution (similar to yours) to the problem they have.
If you find this evidence, it proves that:
1. There is an inefficiency present (opportunity is simply inefficiency by another name).
2. It shows that the problem is painful enough that people are trying to find a way to solve it. And..
3. It proves that the customer is aware enough of the problem to be able to see you as a likely solution (so you don't have to educate them).
I used this process to come up with my own successful ideas. One of which being: https://myapptemplates.com.
This video will help shed more light on the above https://www.youtube.com/watch?v=exMoRoaxKtQ
I googled how many software developers there are, then used a calculator to guess what fraction of them might ever buy it, and what that might mean in terms of revenue.
Also, the product solves (sort of) a problem I have.