One way is to ask them to pay you for it (pre-order) before you make it. For the original Pebble I imagine Kickstarter helped fill this role.
This worked for a startup of mine as well. For our first product we spent ~8 months building and polishing something we thought (hoped) people would like. No one was willing to pay for it. For our second product we spent 3 weeks building an MVP, shot a short video and took preorders. This time enough people put up money to pay for it so we built it!
Personally, I hate pre-ordering things but many other people do not and it’s a really practical way to answer the question “Will people actually pay us money for this thing?”.