The ideal way to validate it is to be so immerse in the industry/problem such that you understand most of the pain points. Do the job that your ideal customer is doing. Attend the conferences they're going to. Visit the forums they're visiting. Read the websites they're visiting. Analyze the reviews they're posting in TrustRadius/G2Crowd. Look at their Linkedin profiles to see what their day to day involves.
I think this is a great reason to have a co-founder. I co-founded a company with a domain expert, and her ability to get in front of and understand our target audience was way more important to our startup success than my development ability (and the equity shares reflected that). I mean, we needed each other, but she was far more unique than I was.
The alternative for a developer is you immerse yourself into a different domain, or you build tools for developers. Both of which are good paths, but the former is far more time commitment and latter is more common (and thus more competitive).
It's called an inducement for a reason.
Too many "product people" or "visionaries" or whatever just don't know how to do user research and validation.
Too many people also have no idea how to solicit unbiased feedback and too many rely on asking questions and not nearly enough observation or testing.
So its critical that you find a good sponsor/client in the early days. One that you have easy access to and you work together with until your product gets to the point where its utility is obviously apparent to new clients. And work it from there.
When your working with your first client they will inevitably drag you down into the small details (improve this/ make this). I call these the "meh" features. What you need to do is focus on analysing the underlying reason why they want the "meh" features and work on eradicating that reason. If you crack it, you get a "wow" feature. And that brings traction.
For example, we designed a financial platform for schools in a developing country. Finance operations in schools is complex with varying chains of manual processes. Our "meh" features were digitising these processes, improving each individual task. Our "wow" feature was to completely automate the end-end process for a small subset of schools with similar requirements. Instead of making their tasks easier, we just completely removed the need to do them. After that it was pretty apparent what we had to do.
For instance, cold call companies and pretend you have the best version of your product on the phone or create a landing page that offers your "future product", buy PPC ads, and get people to fill out a lead gen form.
In both cases, once you talk a little bit, let them know the product is still under development and ask if they'd like to be contacted again once development is done.
Though I have had good experiences talking to different industries because it could be as simple as a 20 min phone call.
I find that if people are really facing problems that they want to get rid of and someone is genuinely interested in solving it for them, they are usually open to chatting. It does take some effort to reach out.
I have had success talking to 30-50 individuals before I come up with a solution and customer development is also a skill that needs to be develop .