How to effectively validate an idea

They say that one needs to validate the idea before building the product to avoid wasting the time on something which does not worth it. I was thinking about it. When they say "Validate the idea" what does it actually mean? What are the different ways/techniques for validating the idea. Ask to random people and take their opinion? But that might not help a lot considering the experience, domain and expertise required would need specific audience in that area. One might reach to existing customers(if they have) but this would only be helpful if the idea is related to the product which the customers are either using or they have the experience dealing with related domain. But then how to reach the right ones, could social media be any help here? but I think no one gives much attention & time there to help you validate your idea. Analysing related products in the market could be a bit helpful but it might not give you the exact information that could help you validate your idea. What do you think? What is your way of validating the idea. Please share.