I've spent more than 20 years helping people understand their customer's needs, and it starts with listening and observing.
First, you either need to invest time or money...there is no other way. I would suggest time, and I would refer you to the Lean Startup methodology. I good online resource is leanstack.com, Ash Maurya's site.
If you have an idea you think might solve a problem for someone the first step is to validate that who you think has the problem actually does have the problem, and they have tried to solve it, have spent money trying to solve it, but still have the problem.
So identify potential customers and go talk to them about the area you are interested in. Don't pitch them a solution yet until you have validated the above hypothesis. Listen to what they say, watch what they do.
This is a starting point, so check out Lean Startup or Business Model Canvas.
If you want any additional details, give me a call