Even with a geiger counter, you wouldn't be able to tell, as the longest half life of any of the isotopes created was ~12 hours, and the most common one was 22

Even with a geiger counter, you wouldn't be able to tell, as the longest half life of any of the isotopes created was ~12 hours, and the most common one was 22 seconds. There's basically no radioactivity left in these dimes after a few hours, let alone 50 years.