It seems likely that Snape told Draco to set a snake on Harry, to test if he was a Parselmouth. But where would he get the idea Harry was?

I don't think Snape knew - his response to it is a change in expression - where before he was enjoying setting Harry up to be frightened by a snake, now he responds as if he hadn't expected what happened and he is working out the implications:

Snape, too, was looking at Harry in an unexpected way: it was a shrewd and calculating look..

I suppose this could also be read as Snape having found out what he expected, but the shrewd and calculating look comes after he tells Malfoy to cast the snake spell.