Off the top of my head, here are some places to start:
Google Tools (Trends + Search Console)
See if there are any Facebook Groups on the idea/topic/thing.
Try to understand Who might need the thing your idea is about. Where do they hang out? How do they describe themselves?
Using result of Step 3, look for Facebook Groups dedicated to helping those people (this step is different from step 2. Here, you're searching for people, not the specific thing/idea/topic).
Using result of Step 3, look for sub-Reddits dedicated to those specific people.
See if anyone has asked questions on Quora relating to the thing/idea/topic.
Repeat step 6, but on HN
Step 6, but this time on Reddit
Step 6 again, but this time you're searching in YouTube. Have folks been making videos on this topic? How active have the comments been? What is the prevailing sentiment in the comments (like: omg, thanks, this helps me so much)?
Next, search on Twitter. Are there accounts dedicated to this? Are there hashtags surrounding this? Have folks been tweeting about this, looking for solutions?
Pop keywords that relate to your idea into Answer The Public (https://answerthepublic.com/). Look for trends.
Using results from step 4, look to see if anyone has been asking questions in those Facebook groups that relate to your idea.
If no luck in many of the steps above, then pop into those Facebook groups, and start looking for patterns in what problems folks are trying to solve. Especially look for problems where people are spending a lot of money/time/energy to find a solution, even if the solution is less than perfect.
When you search about this idea on Google, do you find how to articles? Discussion forums? Blog posts?
Search on Medium. Have people been writing long-form think pieces about this idea, or the problems surrounding this idea, or about the people who might use this idea?
Look on Fiverr. Are folks offering gigs that offer to do a process that your idea actually eliminates? Look at the number of purchases, and look at the reviews. (Thats a good sign that your idea can make $$)
Look on amazon. Go to the Kindle store, and look for recent books authored about this problem space. Are there any? If so, scroll down to the 3-star reviews. What do those say?
Similar to 17, but if it applies to programmers/programming, look on egghead and udemy and see if anyone has created a course related to this idea. Does your idea eliminate a long process, or make it significantly easier? Is it a no-brainer that your idea makes it easier?
If the idea is maker-related or founder focused, look on indiehackers, and discuss.bootstrapped.fm and see if anyone has been looking for solutions to this problem.
Look on Indiegogo and Kickstarter to see if there have been any products created in this space, or to solve the same need. Look at how fast they funded, how many backers, and other engagement signs. (You're looking for folks who are rapidly stepping up to say "omg here, take my money")
Hopefully your idea solves some existing problem. That's the easiest thing to tackle becaue then you don't have to first spend a lot of time educating folks on why they even have this problem they didn't realize they had.
In all these steps, keep an eye out for rabid, rapid engagement. Keep an eye out for adjectives and emotional adverbs used by target customers that will indicate they have a high desire for the problem to be solved.
And at each step, you can reach out to individuals and ask about how they've attempted to solve this problem in the past, how much they've spent to solve the problem, and how often they need this problem solved.
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