There are several techniques but they all involve validating with potential users.
A good approach is creating a realistic prototype of what your are trying to build, show it to users and see how they feel about it. Then use that experience and the gathered insights to improve your prototype until it's solid enough to be confident about building it.
This post we wrote can help: https://lightit.io/blog/i-have-an-app-idea-what-should-i-do-...