如果数据库中已经存在一个值，如何显示错误消息?

I have a page where users can create topics via ajax. Before a topic is created I check to see if it already exists in the database. If the topic exists in the database then I want to show an error message via a div box before proceeding to submit the new topic. Everything works fine. But I can't figure out how to make the error message appear only when the topic already exists in the database.I have a page where users can create topics via