I found the frequency of contamination of scrap metal with radioisotopes to be disturbingly high (this article lists several more cases in addition to those in the wikipedia article: https://slate.com/technology/2013/05/asos-investigation-into...), and we don't seem to be able to reliably detect contaminated goods at the border.

Reading these articles inspired me to build a geiger counter to confirm that none of the metal items I own are contaminated. I used a kit (from https://sites.google.com/site/diygeigercounter/ , but other sites sell them as well) to build a detector that can reliably measure gamma for about $60. Tubes that can detect alpha (e.g. from contamination with Americium-241) as well as test sources (with comparable activity to an ionization smoke detector, i.e. safe and legal for anyone in the US to own) for both alpha and gamma are slightly more expensive but readily available.