Ask if they have a guarantee at all before even buying the fish. That's the best approach.
Some mom and pop shops do, but a lot don't (that's a tough order to fill when you're a small business).
Every chain store has a guarantee.
My LFS is the best in the SGV. They have great prices, including quantity discounts (i.e buy 4 of the same fish for cheaper price). Out of 10+years of going there, I've never seen a dead fish, so it's understandable that they have no guarantee. There's a sign in the store, and it's on the receipt.