[Q] How to discover whether or not there is a market for your App idea?

[ryno2019]

Most people have heard of "lean startup methodology" which can be boiled down to: "don't build any more than you need". In practice, how do you "test" to see if your App idea might be successful without building, launching and marketing? Are there effective ways of "testing" the market to see if you've got a good idea before you build?