Currency has specific design patterns that were specifically created to be easily picked out via computer algorithms, and it has to be that way to avoid false positives. Governments have 100% control over the design or currency so it was an easy solution. But counterfeits still exist, you aren't going to be able to prevent someone from using lenses, etching chemicals, and plates to copy currency. And it only works because micro-jet printers are black magic that you aren't going to replicate at home. Whereas a +/- 0.002" milling machines are a comparatively easy construction, especially when you can make them with parts in stock from McMaster-Carr.
Firearm receivers are about functionality not easily reducible to a specific test or single thing to look for. AI won't work, as there is just too much variate and chance of false positives. In addition and adversarial AI can easily generate a design the tricks the first one by slightly changing non-functional parts of the design. Also even if perfect, you could print a 70% receiver and the jigs needed to finish it with hand tools.
Best method is to blacklist the hash signature of known firearm containing files. It's not going to stop someone that's determined and knowledgeable (and you never will) but will be enough of a hurdle to prevent every tom, dick, and harry from doing something in a hurry they might regret.