How to test out demand for this idea?

Depend on how much you willing to invest to do the validation.

The methods vary due to many factors such as the nature of business, the ease of collecting data, quality of the information produced, set of assumptions made, etc.

From a simple survey to a structured survey ...
From an empirical approach to a scientific approach ...
From a DIY manner to engage professional ...
From concept testing to sample testing ...
From evidence-based to belief-based...

Should you select one or have a mix of these?

Hope the above help.

If you need more clarification, please contact me without obligation

Hello, I work with many early start ups and can provide the tips you need. Vendasa.com for example provides while labeled marketing products and they target businesses in the internet marketing industry. My recommendation would be to connect with specify store owners, and provide them with this opportunity. Please contact me so I can create a DIY marketing plan for you and also spend at lease 1 hour going through the sales process of how to cold call, and email these businesses.

Hi,
I love the suggestions and ideas that you have gotten from the experts. If you don't mind could I ask you a few questions?
a) Have you been building a portfolio of your products?
b) Have you been selling or asking persons to sample and ask for their comments?
c) Where are your testimonials?
If that is so then you have the confidence you need to match the market. You must first be confident in you and the product and then carry that confidence with you in everything that you do. Even if your supply does not meet your demand right now you are positive about the projected end result will. When your proposed clients says." Tell me about your product." you must be sold on it.
Persuasion: People will buy if they are persuaded. And you cannot persuade if you have not bought into the product.
After you have done all your marketing strategy..be confident that it will sell.