Two common things give it away: a lot of times the poster’s comment history will say things that contradict his/her own story. For instance sometimes they’ll write a viral “am i the asshole” post where they are a thirty-two year old woman, but a few weeks ago they commented on another post claiming to be a 17 year old boy.
The other thing is that often the post will respond to questions in the comments, and that can give them away. The person will have a post that seems convincing, but a commenter will pick up one detail that doesn’t seem to make sense and they’ll ask OP about it. Often OP’s answer isn’t as well thought out as their post, and that leads to more questions, leading to more holes being exposed in OP’s story. By that point, however, the post is already on the front page.