Ask HN: How to validate a business idea before building the product?

I'm working on a B2B product in a specific niche (http://www.untab.io). Before spending a ton on development or marketing, I'd like to get some indication of whether this is a real problem beyond my own circle of friends and colleagues.

There are some standard ideas on how to find potential customers - e.g. through social networks, personal connections, etc, but I'm curious what has worked for others. Do you cold call? If so, how do you find the right people, especially in larger companies? Do you use paid advertising? Post on social media?