If people are universal in their opinions that something isn't going to work, you then have to assess their credibility.
If you decide that the problem is they lack some specific insight that you possess (and you're really sure about that fact), then in combination with their universal negative position, I believe you can be confident that you're on to something. There's a tendency for strikingly new things to be laughed at, or otherwise denounced as absurd; once you notice that effect at play by people, and you can reasonably confirm your own thesis that you know something they don't, it's a great indicator.
If you decide that the problem is they lack some specific insight that you possess (and you're really sure about that fact), then in combination with their universal negative position, I believe you can be confident that you're on to something. There's a tendency for strikingly new things to be laughed at, or otherwise denounced as absurd; once you notice that effect at play by people, and you can reasonably confirm your own thesis that you know something they don't, it's a great indicator.