Write a detailed summary of your novel or story. This includes key plot events, information about your characters' background and setting. Let the plot summary sit for a few days, then go back and re-read it. If it sounds like something you may have read before, do a search on the Internet. Input key words into a search engine and see if anything relevant to your story appears.
Join a writer's workshop. Having another set of eyes look over your fiction can help when it comes to avoiding plagiarism, as others may have read something that sounds similar to what you have written. Visit your local library to get started, and ask if they have any information on writers' groups.
Use plagiarism software. Programs such as "The Plagiarism Checker," "Plagiarism Detect" and "DupliChecker" can help you find words or phrases that have appeared in text somewhere else. If a match comes up, you can adjust your own text so it is not the same as another writer's.
Let your fiction rest for a while before going back and reading it. You've spent a lot of time writing something, and you need to be able to look at it with a clear head. Stepping away from your fiction for four to six weeks and then going back to re-read it will help you to see if it sounds similar to something you have read before. If it does, do an online search for key words and see what turns up.