Indian food is not one cuisine but dozens. The mustard-oil based fish curries of Bengal are nothing like the coconut-and-chili dishes of Kerala. The tandoor-bread culture of Punjab shares almost no ingredients with the rice-and-sambar meals of Tamil Nadu. The first question when choosing an Indian restaurant should be: which region?
For North Indian cooking (the style most familiar to global audiences), look for restaurants that make their naan and roti fresh to order. Pre-made bread kept warm under a lamp loses its essential texture. A good tandoor oven is not a small investment — its presence means the restaurant is serious about North Indian cooking.
South Indian restaurants should offer at least five or six varieties of dosa (thin fermented rice crepes) and make their sambar — the lentil-based vegetable soup — fresh daily. Tinned or frozen sambar has a particular flatness that's immediately recognizable.
For reliability outside India, look for restaurants where the kitchen is visibly Indian-staffed. The cuisine requires knowledge that isn't easily transferred through recipes alone. The spice-building process — blooming whole spices in ghee before adding aromatics, then base sauce, then protein — is instinctive to someone who grew up with it.
Lunch thali menus (one price, unlimited refills of multiple dishes) are the best value in any serious Indian restaurant and also the best way to understand the kitchen's full range.