The best way to tell if salmon is done is by using an instant read thermometer. I use one of these every time I'm cooking fish. I like to remove the salmon from the heat when it reaches about 135°F. This accounts for 10-15° of carryover cooking after it's removed from the heat to reach medium rare (145°F). If you don't have a thermometer, make sure the salmon is firm to touch and flakes when you cut into it.