the best thing to do is become familiar with foods that are rich in omega-3 fatty acids. plant sources (walnuts flax vegetable oils like canola oil) have alpha linolenic acid (ala) and animal products especially fish contains docosahexaenoic acid (dha) and eicosapentaenoic acidand (epa). sometimes omega-3 fortified foods like milk and eggs will declare how much they have on the label or you can call the company for that information. for more on shopping for omega-3 sources see this webmd article.
where can i find out what quantity of omega-3 fatty acids is present in a particular food? it is not listed in labels.