If you buy it in a grocery store. Go to a good sushi restaurant, whose reputation is staked on what they prepare, and you're getting what you order. No self respecting Japanese chef would lie to his customers. You can verify it by having the same fish elsewhere, or in other countries. Shark is shark, eel is eel, etc. In the really expensive sushi joints, you can watch them cut the fucking fish up right in front of you.
Commercial sushi will say right on the package that it's imitation whatever it is. And I'm fine with that. $6 California rolls don't have to be authentic to me. Just tasty.