I buy fish oil capsules at the drug store or WalMart. The same ones that humans would take. I trust the quality of those more than I do products made for dogs. Some dogs will swallow the capsules. I prefer to poke them with a pin and squirt the fish oil over their food. The back of the label should list the amount of EPA and DHA in each capsule (sometimes it will be the amount in two capsules). Those are the really beneficial fatty acids in fish oil.
If I were going to give a product made for pets I'd probably use
Grizzly Salmon Oil or
Nordic Naturals. Grizzly has been around for years and gets really good reviews, and Nordic has a very good reputation for the quality of their human fish oil supplements.
And your vet may have something to recommend.