Validating before building - i will not promote.

I read about this a lot. People tend to build before actually validating the idea, which i think is fine, but many people say that you should validate the idea before building. But the problem is when you don't already have a product it's really hard to talk to people about this idea without showing a real product, because sometimes the idea seems vague on text but it's really good in the actual product. What do you think about this? and do you actually have a way to validate an idea before building (especi