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")

Make website. Advertise website on Google or Facebook or whatever. Have pretty screenshots about what the idea is. When people click the sign up button or try to enter their credit card info, it crashes.

Another alternative: put up flyers. When someone calls, say that the service is currently fully occupied. (works great for car rentals, cleaning services, etc)

After validating, you might want to shut down the advertising.

Depends on the idea itself. If it's a business idea, talk to people in your imagined target group and validate that the need is real, and that the way you want to solve it isn't wrong in an immediate sense.

My top 2: 1. Interview customers to discover the right segment and if they care enough about the problem you want to solve. 2. Use a landing page to validate the solution and marketing channels before you build it.

One of the main reasons why a lot of startups fail is that they produce their product or start offering their service without even first making sure that the customers in their target market want what they are offering. Therefore, before doing anything else, make sure that you understand your customer’s wants and needs without copying what some other company is providing or has attempted. Here I read an article might be helpful: https://cabstartup.com/why-startups-fail-and-how-to-run-a-su...