The signs of becoming a parseltongue are quite simple really. You may hear voices others don't, or can surprisingly tell snakes and serpents orders. This may mean you are related to Salazar Slytherin, who was one of the original founders of hogwarts, and created the slytherin house. Salazar believed that only pure blood wizards and witches should go to the school, so he left Helga Hufflepuff, Godric Gryffindor and Rowena Ravenclaw to look after the school by theirselves. It is a very rare ability to be able to speak parseltongue.