Search job boards for job openings that list the technology as a requirement.

Not sure that is a reliable indicator. How many times have you seen job openings that list under required experience years of Java, c++, JavaScript, angular, Java script, HTML, react, nosql, mongodb, and postgres all on the same line? The majority people writing those job ads have no idea what they're asking they just dump the list a few stakeholders gave them and these stakeholders will put anything they can think of in there, including technologies the company has used once 10 years ago for a small project that is long dead.

I have thought of getting on BuiltWith, AlternativeTo, and StackShare. There's probably a way to find small companies doing this (and the job posting tip, which is interesting) but I haven't been as proactive as I could be, I guess.

I think I first heard about it on HN, but there's an attempt at this called (StackShare)[https://stackshare.io/]

Crunchbase has a Built With section. I'm not sure how its constructed, but I'm guessing it only looks at what it can determine from a company's site. I suppose github could be another source.