They ask innocent, shallow questions that don’t seem to get much information from you: last name, home town, schools, diet, temple where you worship. Add that to visual clues like whether you have a string around your chest, where you have a dot placed on your face or how big the dot is or whatever, etc. Put all those small, innocent little bits of information into a big picture and it’s basically a logic puzzle of finding enough clues to narrow down the caste.