How to assess the impact of an academic paper in the industry
This answer deals primarily with engineering products, I'm not sure if it applies to say, the software industry. Most research papers will present some results, but not industry-ready solutions. Most industries will not have full fledged research teams to scan scientific literature and work on translating it into products. So there is a gap that an interested industry needs you to bridge. Therefore, when an industry finds something interesting, they will contact you, as has already happened with you. The chances of many companies using your research without your knowledge is slim.

If you are interested in application-oriented research, you can increase the chances of this happening by attending certain conferences which have industry delegates, and proactively interacting with them. This can lead to collaborations, sponsored projects and a general understanding of their requirements. As you said, there is not much of a public paper trail, so human interaction is the only way.

To know what impact other people's research has on industry, you could search for papers sponsored by those particular industries (acknowledgement section for example).