Calculate the amount of omega-3 fatty acids in fish by searching the United States Department of Agriculture National Nutrient Database. This site will allow you to find all the nutrition information in almost any food you can imagine. You choose the amount or serving size and the program will list all the nutrients in the food.
The two important omega-3 fatty acids found in fish are called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). They are listed under the "Fatty Acids - Total Polyunsaturated" heading. DHA is listed as 22:6 n-3 and EPA is listed as 20:5 n-3. Add them together to calculate how much EPA and DHA you are getting from the food.
Here's an example. If you search for salmon, you will be presented with a list of different types. If you choose a three ounce portion of wild, raw coho salmon, you will be presented with a list of all the nutrients. The 22:6 n-3 is equal to 0.558 grams of DHA and 20:5 n-3 is equal to 0.365 grams of EPA. This gives you a total of 0.923 grams (923 mg) of omega-3 fatty acids.