We can't. The only way we can is through logic. Petey is a huge Giga Bowser/Bug eating plant with extremely strong jaws which he probably uses to catch huge prey. Then it would make sense that he uses that acid to blind his prey and then bite them.
And because of the size of his prey and himself, he would need to get all the energy he can get to mantin himself, so he would need to digest every body part of his huge preys to do it, so he has a potent acid to digest everything, even the bones.
Also, the potency of his acid is plausible considering there are lots of lifeforms capable of digesting all kinds of trash, plastic and glass. Common example: vulture.
This is of course, an extremely complicated and far-fetched theory, so don't mind me.