There are a couple of ways to do it. The first method is the old school Chinese way (aka the way I was taught by my grandma). Poke the thickest part of the fish with a chopstick, all the way through. If you don't feel any resistance, then the fish is cooked. But this method works best with experience. The second method is by using a fork and poke, at an angle, into the thickest part of the fish and twist slightly. If the flesh flakes off easily and the colour is solidly white and no longer translucent then the fish is cooked.