The generally accepted answer is that the only way to know for sure that people will buy something is to get them to pay you money or get them to commit to pay you money.
This falls broadly under the concept of the "lean startup" where you focus first on proving you can sell what you intend to build, and only build it after you have cash commitments from customers to buy what you intend to build.
The logic behind lean startup is that it's far more likely your startup will fail because no one will pay (or pay enough) for what you want to build than it is likely to fail because you can't actually build the thing you want to build. The later case is of course possible, but in practice far more startups fail because of lack of sales than from lack of technology.
The generally accepted answer is that the only way to know for sure that people will buy something is to get them to pay you money or get them to commit to pay you money.
This falls broadly under the concept of the "lean startup" where you focus first on proving you can sell what you intend to build, and only build it after you have cash commitments from customers to buy what you intend to build.
The logic behind lean startup is that it's far more likely your startup will fail because no one will pay (or pay enough) for what you want to build than it is likely to fail because you can't actually build the thing you want to build. The later case is of course possible, but in practice far more startups fail because of lack of sales than from lack of technology.