Read labels. Look at the details of the supplement. Many fish oil supplements actually have very little omega 3's in them, & it's often difficult to tell what else is in them. Epa dha are the omega 3's most studied/shown to be beneficial, but often r not the main part of the fish oil supplement. As unregulated products u can't know 100%, but at least make sure they list the good things as the main components.
Omega 3 Fat. The problem in industrialized nations is too much omega 6 fat, from soy, corn, cottonseed and safflower oils. These fats oxidize themselves through the same process that makes oil-based paint dry--and increases the need for anti-oxidants. Deep sea oily fish is the best source of omega 3 fat, but canola oil also supplies some.