On a small scale your ability to sell the value of your idea ultimately must be validated by someone’s willingness to pay you. That is the first thing to work on, does your idea cause people to pull out their wallets and pay for the product?

But what if the product/service doesn't exist yet?

When you are going through the process of validating if an idea works you must create some type of prototype. This does not need to be the finished product or even something that is tangible. For example you can act as if the service/product already exist and create a website around it. On the website you can inform people to submit their email to be notified about the product/service.

Many email subscribers is an indicator that there is interest and you are building an audience to market to in the future.

Also asks yourself these questions:
What problem does this idea solve?
Who is my ideal customer?
Who are my competitors?
Is my idea scalable?
What are the benefits of the idea? (How does this make people better?)
What are the features of the product/service?