1) Use a cooking thermometer to check if the fish is cooked by inserting the probe into the center of the thickest part of the flesh. Cod and halibut should be around 130°F to 135°F (other fish may vary, so check the link to see what temperature works for you). 2) Use a fork to gently prod the center of the fish; if it appear opaque and flakes easily, it's done. If it looks translucent and resists flaking, it needs more time.