If the fish on the menu isn't caught sustainably
"Seafood Watch is a great resource, and a great app. Users can recommend restaurants that have a sustainable seafood program. And restaurants can petition to get on the app by sending them menus. Granted, it's not always the most definitive list. I've worked at restaurants that have been on and not on the list, and both places sourced as sustainably as possible."
The menu doesn't say if the fish is wild
"If the menu says it's salmon, but doesn't tell you if it's wild or farmed, that's a red flag. Nobody wants to put farmed seafood on the menu, and they certainly won't highlight it. If it's wild, they're going to tell you."
The fish is served without skin
"I like to see restaurants serve fish skin-on -- you can look at it and tell what it is (and what it isn't!) when you're eating it. When you're getting a pan-roasted piece of fish, you want to see the skin on. There are some exceptions with fish where you don't eat the skin, like certain flatfish. But if someone tries to sell you striped bass or black bass and it comes out and has no skin, that's a red flag.