I've been told that the top should easily flake with a fork (this is when baking skin-side down). Is there any fish for which that is not the case? And what should the middle look like with which types of fish? Or would it be better to go by thermometer? In which case is it usually, "Once the middle hits 140F, it's done"?
I have a simple way to do this for all types of fish.
You simple find a chopstick and insert the chopstick in the middle of the fish toward to the fish head, which means it's usually thickest part of the fish.
If you can easily insert the chopstick through the fish, it is usually fully cooked. If you find it difficult to insert the chopstick or feel your chopstick is stuck when there bone is, it generally means uncooked. Otherwise, you can also feel the temp of the chopstick and it should be warm in the middle the part you insert into the fish.
Inserting the chopstick shouldn't damage your fish as the hole is small.