This, exactly. It's less that there is true evidence unless people save their plurks and chat conversations, and more that you just slowly figure out she's a backstabber and a liar. At first you think she's just venting about people, or has legitimate complaints, and you think nothing of some of the things she says. Then you realize it's way worse than that, and she's saying awful things about everyone to everyone else, and expecting them to never end up sharing stories with each other.

As the anon I'm replying to says, it's hard to tag with someone you know is lying to your face by acting sweet and innocent when they're really a petty, possessive, backstabbing bitch.