When you wet or dry cure fish does all of the fat come out with the water?
It should be mostly just water, very little fat. Water is released pretty easily, fat not so much, especially without grinding things up. Most fish isn't that fatty to begin with, anyway.

In any case, fat and water don't mix without help, so just look at the liquid. If there's fat there you'll see it floating on top. I'm guessing with fatty fish there might be enough for a thin shimmery layer on top, but I doubt you'll ever see very much.