I had discussions with longstanding professionals in the field so as to understand where, how and if the business concept slotted into the current framework, evaluating it's USP by their overview of the status quo. Professional insight is worth its weight in gold. I found getting validation for the idea is very different than proof of concept which is the moment when in describing your service or product, the person enthusiatically replies with a wish to have what you're offering. Listen, learn and be flexible.
The best way to validate anything is by taking action and testing it. If you offer a service, find 1-3 clients willing to work with you to test your ideas and provide feedback. You can offer your services for free or a reduced fee as the value you receive is in their feedback and building testimonials for your organization. Good luck! Connect with me for further help/discussion.
List them down and discuss with fellow peers whom you might join to start the company or if you begin the company alone then check research out there that might be similar to a company you are launching. Because it with give one a rough idea what to expect in a challenging market.
Use a survey - some people you know and some you don't - give 5-10 questions about your idea without giving it totally away. Use T/F or multiple choice answers. It should give you enough info to see if the plan is valid or not.
put up a google adwords ad with the idea name and the benefit. link a landing page to it and measure the click thru rate. on the landing page say that your product/service is in the making and that you can send first tips if the visitor leaves her email address. count 20% of those who leave the address as future customers and do the maths
Interviews and surveys before you even have something to sell.
Only after you've validated your idea/your users/your customers/your revenue stream, then spend time and money on "building" your Minimuv Viable Product (MVP) and then test it with your users/customers/revenue stream etc.
To the best of your abilities, identify your target customer, and go talk to them. Ask them about your proposed product or service, and get feedback on your assumptions.
If you live in an area with a mass transit system, I recommend going there to find folks to chat with! People who *just* missed the train have a few minutes, and most (at least here in Boston) are very welcoming if you're friendly.
Find groups of people who you believe could become customers and do some market research with them. And ask them who they know who might be willing to help you with your research as well.