You also have to know the "character" of the fish, as if you didn't need any more stuff to tell fresh seafood from bad - lol!!
For instance, for the vast majority, if not all, fresh raw fish, there should be absolutely NO flake separations. Flake separations are a really telltale sign of old fish - particularly salmon & other large filets.
On the other hand - fresh cod - even VERY fresh cod - naturally flake separates, even when boat fresh, so your best bet with cod is to look for white moistness. If the filet(s) are brown around the edges & look dry, pass them by. Cod should look moist & fresh (whether fresh or frozen-thawed, which is how most of us usually get it), & flake separation is just fine.
Flake separation in salmon or really any other large fish is unacceptable & you should pass on it.