There is no absolute way to distinguish between marine fish and freshwater fish by eye recognition alone.
From some life experiences, marine fish are more grotesque than freshwater fish.
Secondly, marine fish have no intermuscular spines, while most freshwater fish have intermuscular spines.
Finally, you can judge according to the kind of fish (that is, judge what fish it is according to its appearance, so that you can know whether it is a freshwater fish or a marine fish.