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-...
You need to hire a team that can understand your vision and combine it with its technical expertise in order to create a successful product. It is hard to find that on Fiverr. Signing a Non Disclosure Agreement is usually a standard practice and will help you protect your idea.
Full disclosure: I own a software development agency that focuses on start-ups, so we can help with validation, and product discovery apart from development.
If interested you can check us out at: https://lightit.io