Sell it before building it.

If there are no buyers - validation done.

If there are buyers - refund money, apologize for delay, send X% OFF coupon and start building it.

Maybe a bit controversial but works with many types of products or services.

(About 15 yrs ago I had an idea to combine all esoteric spiritual ideas that belongs to certain fragmented discipline into one place and sell it as eBook. That exactly what i did and was very surprised to give 4 refunds within short period of time, before putting it "off market")