It's exactly the same. If nobody tells you that fire burns, how can you possibly know without trying it? How can you possibly know that lilies can poison your cat without trying it? Through common knowledge, through people telling you, through your parents warning you.
I just remember being told as a child by my mum not to let our kittens near the lilies she had outside in a planter, and clearly quite a large number of people here know about it through some way or another. Do you think they all tested lilies on their cats to find out?
I used the words 'fairly' because obviously not everyone is from a cat loving family and wouldn't therefore have any reason to know. I wasn't saying that EVERYONE KNOWS THAT DUUURRRR obviously.