How do I find out what's wrong with a 4chan post?

The first paragraph of the 4chan guide to 4chan for beginners (saints save us) says:<br><br>>Look for a post that has been deleted.  Ask what was in the post that caused it to be deleted.<br><br>but it doesn't tell me how to do that. After a post is deleted (and the text is gone), how do I find what the post was made of? Is there a link? Is it in the source code somewhere?