The classic signs everyone mentions (English menus near Champs-Élysées, photos of food around Trocadéro, aggressive hosts near Eiffel Tower) are obvious. Here are subtle tells that saved me from mediocre meals around République:

Check wine prices. Tourist places near Louvre charge €8-12 for house wine that should be €4-6 in proper bistros. Look at dessert menu — If it's all commercial stuff (mass-produced tiramisu, factory cheesecake), the rest isn't made fresh either.

Most importantly: watch staff during service around Saint-Germain. In real restaurants, servers know menu and can answer questions about ingredients. Tourist trap staff near Notre Dame just point at menu items and rush you through ordering.

Also, restaurants completely empty at 8pm but packed at 6pm around Marais are feeding tourists who eat early. Parisians eat late, especially in République and Belleville areas.