I don't have the skills of the person who decoded the signal... however, if you get your ham radio license and start playing with the various digital modes, you'll start to recognize the various sounds of different modes. An APRS data packet [1] has a very unique sound. Same with with RTTY [2]. Our brains are really good at pattern matching if they're exposed to a pattern enough times. Obviously, you'd still need a computer to decode those signals, but the "sound" of the signals can give you a very good headstart.