First things first. I fundamentally agree with what Dan and John have both said, and I have personally taken advice from Dan on a very closely related topic in the past.
I also mentor startups to avoid wasting time like I did in my startups. So beware, because not testing correctly at this phase of the idea is an easy way to cause yourself lots of future pain.
You have three big concerns with any new idea, whether you are a startup or a product manager at a corporation responsible for a new product launch. In order, they are:
1. Who? (is my customer)
2. How painful? (is their problem)
3. What? (will they give me to solve it)
The answers to these questions are obviously all related. The best way to figure out the answers to these questions is to talk to people and ask them non-leading questions (lots of info out there about how to do that. If you want to avoid reading too much, we could chat about how to avoid this.) How do you actually do this?
1. Write down the problem you think you are solving and who you think has it.
2. Talk to those people and ask them what their biggest issues are. Get them to explain any problems that you think might relate to the problem you are solving.
3. Ask them to give you something to solve that problem (essentially do what Dan and John are saying, try selling your vision of the solution). Take careful note of how they respond to the way you describe it.
It is important to note that even if the problem isn't a BIG problem for them, meaning it is not in their top 3 problems, that you still may be able to build a business out of it. However, if you are solving what is consistently their number 10 most important problem, your sales cycle will be longer, your growth will be slower, and you will learn how to patiently build a business.
Cheers! And good luck.