Easiest way is probably to get the pink salt from the Himalayas. The health claims around it are bogus, but as it's mined from the mountains (and usually sold in a highly unrefined format) it's fairly unlikely ocean microplastics have wound up in it. It's also readily identifiable in a way white chunks of salt aren't.

> In another indicator of the geographic density of plastic pollution, microplastics levels were highest in sea salt, followed by lake salt and then rock salt.