If you are not sure about a specific fish, and you cannot investigate it with your eyes, you can look it up online or consult a list like this: https://ihic.org.au/common-halal-and-non-halal-sea-foods/
Pro tip: If you are buying seafood which is prepared with ingredients, be sure to check that it does not have alcohol! Because cooking with wine and other forms of alcohol is common. Sushi also often has mirin wine. So you can look out for that as well.