The rule of thumb for fish is to cook it for 8 or 9 minutes per inch of thickness. That varies a little with the fish, the method, and other things, but not much. If you're Alton Brown, you test it for doneness by sticking in a thermometer to see if the thickest part has reached 135F (it's done at 137, but the temp rises a bit after you take it out). If you're a lesser being like us other cooks, you either cut into it to see if it's opaque all the way through, or you stab in a fork and give it a little twist to see if it flakes, or you push on it with your finger to see if it feels firm enough.
When I'm at home in the kitchen, I use a thermometer or a fork, depending on whether or not I can find my thermometer. When out hiking, fishing and camping, I poke it with my thumb. It's all good, so I think it doesn't much matter.