I validate my ideas to people I trust. Some people may be in the business, but I also check out the ideas to non-business people. I am also part of a small group of 'friends' where we can check all ideas that others have
Validating your idea really depends on the product or service to be offered. If the idea is something that you can provide a demo with little capital, then having candidate customers and even friends do a dry run of the product or service can provide valuable feedback. If the idea is more capital-intensive (high tech, biotech, etc.), then a demo is impractical and validating the idea requires market research such as candidate customer interviews, an analysis of the competition, and a knowledge of the industry trends.
Some of the best feedback you can receive is when a potential customer expresses dissatisfaction with some aspect of a current product or service, especially when provided by a larger vendor. Dig deeper into those pain points and see if your idea can solve them.
Finally, just keep pitching the idea, you never know what markets may open up that you originally never thought to address.
Find past case studies of similar ideas that have been put in place and achieved successful results. Using statistics/research and trends that support and substantiate your idea also helps. I think its often a numbers game. Evidence seems to be very important in persuading others to believe that you ideas will work, even if in small measures.
Finding the people that your product or service would be serving and ask! You can also run some adwords testing using a site like unbounce to measure the effectiveness of the campaigns you set up. If it is a service check out Ramit Sethi, he has a lot of good content on this, for a product consider Tim Ferris...
Several ways: Market need (Sales or ready prospects), Competitive analysis (position and validation), and whether you would use your own product.
I typically validate my idea through sales. I know this is a strange thing to say, but it is truly the strongest validation.
I try to identify a lead customer and to get them to pay for my product/service. If they do, I can then close the loop with them and others to see how it worked out. This is great for service businesses with a twist.
As most of my businesses are product businesses (high tech), the cost to build the product is high and it is difficult to sell before it is built. Therefore, for those, I do some market research (are people currently buying the product) in the product and the development path (can I develop a path to the product that people can follow). Once the path is laid out, the only question is are real customers interested in the products along the path; for this, I seek advice from a domain expert (usually my first advisor.
Most other methods of validation are dependent on hand waving and give rise to a lot of interest that cannot be converted into sales. Also, be careful of people who are enthusiastic but don't contribute because they often stick around "in case" you succeed.
That would involve a very lengthy answer.
Your next step should be to contact SCORE at www.score.org
I am a SCORE certified mentor. SCORE is FREE service providing expert business counseling and is a great way for you to understand what it will take to do (or not to do) whatever it is that you are trying to accomplish. SCORE has 13,000 mentors in 348 US chapters. Good luck to you.