Dialect tests. Think about how someone from boston might say “park” like “pahk” vs other parts of the country, or if someone uses “y’all” where they might be from. The way people pronounce o,a, ai, ough, augh type of sounds is very telling. Also phrases are very regional. There are many studies that compile that data. One famous dataset is used in a Times article that is behind a paywall, here are some people talking about it: peabodyawards.com/nytimesdialectquiz/