I suspect sooner or later someone is going to try radioactive currency.

I'm so old that when I went to uni we did a neutron activation of silver experiment in physics class because the half life of whatever isotope of silver was about a minute, so after an hour of lab you could put the coin back in your pocket. I don't know if they'd let college kids play with either a solid silver quarter from a few decades ago (probably worth a couple bucks today) OR a low level neutron source today. I could see picking some very obscure isotope of some obscure metal and dissolving it into the metal of the coin and if your xray metal analysis machine that scrapyards have (a kilobuck ish cost in a hand held device) detects the proper amount of iridium or whatever or a simple neutron activation followed by gamma spectrometer shows the right isotope in there...

Aside from spontaneous emissions and activation reactions, there are some weird cross sections out there. Whatever's irresistable to thermal neutrons thats used in reactors, probably a cadmium alloy, could be the core of a coin, and then blasting boring thermal neutrons that are semi-transparent to fakes but the cadmium core coins would look pitch black from eating all the neutrons. Or play the opposite game and make the coins out of pure neutron-transparent zirconium and only the neutron transparent coins are legit.

Make it look like a normal coin so people can trade by hand, but in any store interaction they could be checked.