I dunno...I still dont' think that he knew that Harry was a parseltounge beforehand.
I think that he just did the snake thing because most people are somewhat afraid of snakes,and Snape wanted, not only to make Harry afraid, but to in a sense humble him by 'saving' him from the snake.
I can't even see how he would have found out before. Harry had only ever spoken to a snake once before, and that wasn't at hogwarts, and I think that if Snape knew, he would have treid to accuse harry infront of everyone, or something like that...