Chefs have in-built clocks in their heads, but there is an easy way of telling when fish is perfectly cooked that everybody can use. Quite simply, put a knife into the fish. If there’s resistance it’s still undercooked. The knife will sink straight into the flesh if it’s cooked. Of course, if you’re cooking tuna or salmon that you want to serve medium or rare, then you are looking for that resistance. A perfectly cooked fish will still be just a little pink at the bone. And there you have it, the easy way to tell if your fish is cooked properly or to your liking.