They don't know, they guess, based on some heuristic algorithm. In Google's case, their algorithm informed my son that the name on his birth certificate is not

They don't know, they guess, based on some heuristic algorithm. In Google's case, their algorithm informed my son that the name on his birth certificate is not a "real name", and forced him to pick another one (he kept his real name as his nickname, and chose his more popular middle name as his first name, keeping his real last name). According to http://www.babynamewizard.com/voyager http://www.babynamewizard.com/voyager, his real first name is the 280th most popular in the US. http://howmanyofme.com/search/ http://howmanyofme.com/search/ says there are nearly 10,000 people with his first name, and a bit less than 4,000 with his last name in the US. But Google says it's not his real name.