The easiest way to do this is to use a web service like cron-o-meter or fitday to track what you eat. Typically you enter foods from a drop down menu or by searching a database (easier than it sounds). You might have to choose between different types of salmon, or different levels of fat trim on beef cuts. The program then pulls information on fat/protein/carb percentages and other nutrients from the usda database and does all the calculations for you.

Some food diaries to look at:

http://cronometer.com/

http://www.fitday.com/

http://paleotrack.com/

http://nutritiondata.self.com/mynd/mytracking/welcome?returnto=/mynd/mytracking

You can also easily search information on different foods at http://nutritiondata.self.com/ from the search at upper right. And they have a handy tool to look for foods that are high or low in certain nutrients.

If you are keeping a food diary, are you not using an electronic device or software/web program to do so? If you are curious about the nitty-gritty, this makes things much, much easier.

For a piece of salmon, yes, the main macronutrients worth recording are protein and fat, so nearly all the energy stored in that piece of fish would be from protein or fat. If you are recording by hand, I suggest making a best guess, or using a points system. Salmon is a high-fat fish (for a fish, not overall), so it may be worth it to record the difference. Recording tuna or whitefish, maybe just record the protein.

I've never used "point systems" for food, but if you are doing this by hand, it would make sense. For e.g., wild salmon has about a 2-3g:6g ratio of fat to protein per ounce, whereas ahi tuna has a 0.5g:6g fat to protein ratio.

Fat: 1 gram = 9 calories Protein: 1 gram = 4 calories Carbohydrates: 1 gram = 4 calories

For example, look at this roasted chicken:

http://cronometer.com/food.html?food=6698&name=Chicken%2C+broilers+or+fryers%2C+breast%2C+meat+only%2C+cooked%2C+roasted

I totalled 160 calories between the fat and protein, they have 165 total.

Then you could just divide... that's 124 cals protein/165 cals total = 75% protein, 22% fat

If you're reading labels, you can see something like this:

Calories: 190
Fat: 10g
Protein: 20g
Carbs: 4g

Fat is 9kcal/gram. Protein and Carbs are 4kcal per gram.

So in my hypoethetical food above:
Fat = 10g * 9kcal/g = 90kcal
Protein = 20g * 4kcal/g = 80kcal
Carbs = 4g * 4kcal/g = 16kcal

So your total calories would be 186 calories (the total "Calories" at the top never quite adds up to the sum of the parts, probably due to rounding and other factors), of which 90 would be fat, 80 would be protein, and 16 would be carbohydrate (I don't count fiber, so if that's broken out, you can subtract that from total carbs).

To get the breakdown, divide the individual by the total:
Fat = 90/186 = 48%
Protein = 80/186 = 43%
Carbs = 16/186 = 9%