It's worth thinking about this in terms of the snowball metaphor.
What's the smallest/simplest/fastest version of your thing you can build that demonstrates that (a) you can make the thing work, and (b) people will pay for it?
Do that, then find customers who will commit to purchase, or investors who know enough about your space that they'll back you to keep working on it.
For hardware manufacturing in the consumer space, pre-orders (Kickstarter etc) are an ideal way to raise funds and prove demand.
For enterprise, you can get pre-commitments to purchase from customers, contingent on the concept working. E.g., Boom Aero came up with a design, then lined up purchase commitments from major airlines, then were able to use that to fundraise to get them through development. The incentive for the airline to commit to purchase is that they will be the first to have your product when it's ready.
For large-scale energy or telecoms innovation, you could build relationships with utilities companies, government bodies, etc - offer them priority access to your technology in exchange for purchase commitments backed by pre-payments or grants, which can then justify private investment.