This is a great question. So simple yet so difficult to answer.
If we were to go about this systematically, the best way would be to write out the reasons behind your belief in this secret, and then compare your reasoning to smart people's. Human brains don't always get it right; sometimes (oftentimes) we possess incorrect or incomplete models of reality and are simply unable to or too slow to adapt. Listing out your reasoning is a way to expose these flaws whether in your logic or in theirs.
Either the people who disagree or you yourself are missing some key information or insight. Maybe they are jaded or scared or risk-averse or never really took the time to fully think about the idea.
That is probably the only systematic way. But, we must account for the importance of courage as well. It's impossible to be 100% sure of any society-changing secret, so also important is the guts to stick with your secret. Everyone could see through the emperor's new clothes but only one person dared to trust himself and risk saying something. Same as in life, many people have known secrets but they didn't risk following through with it.
Again, it's impossible to be 100% sure of anything and we must account for luck. So say some people disagree with your secret and they peg the chance of success at 5%. Well, if you have some insight that they lack, maybe that quintuples your chance of success to 25%. It's not 100% but now your secret could be worth pursuing, even if you fail in the end!
If you are given that genuinely thoughtful response, it will come with the problems with your idea and the reasons that they think there are problems. You can then compare this information to your secret information. If the problems they present match up, and you have plans to solve them, that's great; if they come up with something you had missed, either a different reason or a widely different problem, you may need to rethink the idea.
Of course, this process isn't going to be 100%, and you're still going to have to some faith in that what you believe is correct, but it's a decent process to go through.
I also don't believe you always have to take the non-obvious route, it is just likely to have less direct competition and a higher risk/reward vs an obvious problem.
Its interesting how many different kinds of startup strategies there are. Some take the unexplored route and flail about trying to find a business model or concept that sticks. Others just copy what others are doing and attempt to out-execute / outcompete. The correct strategy probably has a lot to do with the strengths of your founding team. If you are all execution monsters then choosing a well-traveled space and out-executing everyone is a play that could work well. If you have a founder with unique vision, then following him into the Blue Ocean could prove extremely lucrative.