How do you validate B2B startup ideas?

Suppose you find an inefficiency with your company's software development process. You think of a product idea to solve this problem. Before starting a company, what is the best process for validating the idea, making sure other companies have the same problem?

What stakeholders at other companies do you need to talk to and what's the best approach to gain their cooperation to discuss your ideas?

Please respond if you've successfully done this before. Thanks.