No most pest snails can handle more copper then shrimp. So you can't just assume the shrimp will be fine because the snails are fine. If you really want to test they sell test strips for copper, they are hard to read though. Another option is to get a copper remover which might be smart because copper can be in your water supply too. Seachem has Cuprisorb which is like carbon you put in your filter that removes copper.