Your first option is to check the best before date on the packaging. If it doesn't have any packaging, then conduct a smell test. Salmon should have a very mild, slightly fishy odor. If it smells pungent or you can detect the fragrance of ammonia, then you should discard it. You can also check the skin - if it has a milky film on it, then it should not be eaten. If you decide to buy a whole salmon check, the eyes are bright and clear. Clouded over eyes indicate that the fish isn't fresh.