What is the cheapest way to validate a business idea?

A conversation with ten people who have the problem, followed by an attempt to take money from them. Cost: a week. What it replaces: several months of building the wrong thing.

In more detail

Ask what they do about it now and what that costs them. If the honest answer is nothing and nothing, the problem is real but not painful, and a business needs the pain more than it needs the problem.

What to do about it

An idea is worth almost nothing on its own. What is scarce is somebody who will keep going after the first version does not work, which is every first version.

The best ideas usually come from a job somebody already had. You notice a problem because you lived inside it, not because you brainstormed.

A market that already spends money is easier than a market you have to convince exists. Competition is evidence, not a warning.

The mobile app angle

If the idea is a shop, the sequence matters: sell first, prove people come back, then build the app. An app before the customers is a cost with nothing to do.