First, the short answer: Check out the FDA's advisory on mercury in fish & shellfish, then cross-reference the FDA's chart of typical mercury content in fish with your local Seafood Watch pocket guide.
But in the big picture, if you're concerned about mercury in your fish, you should be thrilled about the Obama administration's new limits on mercury emissions from coal-fired power plants. They're often wrongly treated as separate concerns - coal plants pollute our air & water, and there's mercury in fish. That's why reporters let Republicans like Mitt Romney (sometimes joined by coal-state Democrats) bash Environmental Protection Agency regulations on coal-fired power plants in the abstract without connecting the dots to the 1 in 7 newborns at risk for brain damage and learning difficulties due to mercury exposure in the womb.
Grist's David Roberts digs deeper into why the new mercury rules are a BFD.