The fish is cooked when it easily flakes apart with a fork. The FDA recommends an internal temperature of 145 degrees F, but many chefs and cooks aim for 130-140 degrees to prevent an overcooked taste. You can choose what you prefer based on your comfort level with risk, like choosing medium-rare steak or tuna. To read the temperature while it's in a parchment packet, you can poke an instant-read thermometer (I love Thermoworks Thermapen for this!) right through the parchment and into the fish if you want. A little steam will escape but it will be minimal. Luckily, even if you cook the fish to a higher temperature, the texture will still be moist and flaky with this cooking method! We accidentally cooked ours to about 155 degrees and it still tasted amazing. This is one of the reasons why this cooking method is so wonderful!