How do you know how many calories are in non packaged foods (fruits, veg, meat...)

Its easy for packaged foods where all of the nutrition is on the label but how do you know the calorie count for non-labeled foods? For example, I want to make salmon tonight and when I type in salmon to myfitnesspal I see a huge range for calories for the same weight (one says 4oz = 100 cal another says 4oz = 130 cal and another says 4oz = 220 cal). I also looked online online and also see a wide range of calorie counts. Any suggestions?