Usually, you can create a simple landing page for your product or service, with something like “suscribe for early access” then you run an adword campaingn, wich let you see what people are looking for close or simillar of what you’re selling, and if they are interested in your brand they will suscribe for that early access… With a week or two you could check real time demand of your product. And have a pre idea of how much budget you gonna need, inventory etc.