One good method is when you walk into a business. pay attention to how they manage their tasks.

Say, for example, you go into a carpenter's shop, and you see that he writes down what you want built on a scrap of paper. That's your "in". Ask if all orders are stored on paper. "Oh? I'm nikentic, and I build software for businesses. Where do you store these papers? How are you tracking receivables? How are you tracking the progress of each piece? Do you store your customer's email?"

Ask them these questions, in as much detail as you need, using your knowledge of what's possible with technology to make suggestions. You will be pleasantly surprised how much details they are willing to share about their businesses. Ask them if they would pay XXX/month for a system that covers all the major pain points they mentioned. If they would, find other carpenters and ask them if they would be interested. If you get enough interest you might be on to something. Also, how many devs are out actively looking to create software for carpenters?*

*Where carpenters = small - medium companies that are not in the technology sector.