Two things: 1. Smoke tests Set up a landing page, buy traffic and see how good it would convert. You can improve on the copy a little bit (value propositio

Two things: 1. Smoke tests Set up a landing page, buy traffic and see how good it would convert. You can improve on the copy a little bit (value proposition) but you can extrapolate the interest. Also take Google Trends into account. 2. Directly talk to potential customers. "You don't need a product to sell a product".