NPR’s Alva Noe discusses the criteria for determining when variation becomes another language and concludes, rightly, that mutual intelligibility isn’t a very effective measure. The most accurate description of how lay-people actually determine whether or not something is a language or a dialect: “a language has an army and navy.” Do you want to know how linguists determine the difference? We generally dodge the question. We prefer the more neutral term “language variety,” which has the remarkable ability to cover either.
Language News: What is a language?: When Easy Questions Demand Tough Answers : 13.7: Cosmos And Culture : NPR