One Easy method I recommend is to see who else is doing the same or something similar in the Industry. Where are they doing it and how well is the company performing.
Chances are that someone is already doing what you're planning to do and if they are making any money, then they've already validated the idea. If not, talking to someone at one of the companies (potential direct/indirect competitors) will tell you the problems with your idea. Channels to communicate: Linkedin, Twitter, Email.
You can also discover their customers etc and talk to them.
B2B Market Research can get quite complex and I wrote a piece about it here:
You can pick up various tools mentioned in the article to start your research.
Stop thinking in terms of "B2B"!
B2B and B2C are dead terms, or at least should be, and are examples of thinking that lead to anti-behavior.
Everything you do, every decision you make, and every data point you have should be viewed through the Person to Person ("P2P") lens.
At the end of the day, every product and every service is consumed by a person. You are just a person asking a question about how to validate data.
Start with people, ask why from their perspective, and grow outwards from there.
Mamie's advice is absolutely spot on. Validating the idea manually is the cheapest, easiest way to prove the concept. You'll also have much more leverage with investors once they see people are actually willing to pay for the service you want to automate. You can't go wrong developing a strategy based solely on the plan she laid out for you. Good luck!
There are a number of ways to get business customers to try your product, the most common being a waived license fee for scheduled, real feedback on the product (to make improvements). Validation is different that gaining test users, though sometimes what you're talking about is getting serious feedback for your beta product.