How does one know if one can eat raw saltwater fish?
Because you know you have previously frozen the fish.
A lot of shop bought &quot;fresh&quot; fish is likely to have been frozen and defrosted, before sale. Often there is legislation about freezing fish if it is to be eaten raw.
Note that home freezers may not be as effective as commercial freezers and blast freezers so you may need to keep the fish frozen for a few days and it may help if it is sliced thinly so that it cools faster.
Some background info

FDA Detailed Advice That is an 8 page pdf download on parasites in fish. It does open as a viewable tab in my browser (Firefox).
How to Enjoy Sushi Without Getting Infested With Parasites A shorter general answer. Includes reminder you may need to freeze it for a week to be safe.
Rules in my country [England] requiring freezing of fish intended to be eaten raw.