Tyler Cowen has posted a list of dishes he uses to evaluate the quality of ethnic restuarants. For example, he evaluates Turkish restaurants based on the doner kebab:
Turkish: Doner Kebab, taking special care to ponder the tanginess of the yogurt and how it interacts with the meat.
I find that when it comes to “bar food”, you can tell a lot from the fries. Are they the frozen, institutional fries or were they hand cut on the premises? If the latter, chances are they care about the quality of the other things they serve as well.
When it comes to Mexican restaurants and taco places, tortillas are key. It’s hard to take a place seriously if it doesn’t make fresh tortillas.
I’d be curious to know what people consider the best barometer dish to be at Italian restaurants.