People do prefer their fish cooked at different “doneness” and that is fine. However, to test it without cutting your fillet open, you just want to gently push the thickest part of the fish, if it separates along the side of the fillet (you’ll see those white strips of fat somewhat coming apart from the flesh) you should be good to go. In a medium/hot pan it will take one fillet about 4 minutes on each side to cook through.