You want to make sure that your scholarly research makes a contribution to the literature. Make sure that you have something to add to what has been written on your topic. Just because someone has written on your exact topic doesn't mean that you can't write about it- perhaps you will look at different sources, interpret sources differently, otherwise make some contribution to the scholarly conversation.
Steps of Preemption Checking (may vary according to your topic):
1. Look for legal articles on your topic using databases (e.g. HeinOnline, Wexis, etc.)
2. Search for non-legal articles if your topic has an interdisciplinary slant (e.g. Academic Search Complete, general library catalog, Google Scholar)
3. Search for books, chapters in books (Library catalog, WorldCat, Google Scholar)
4. Look for articles that have not yet been published—working papers, works in progress (SSRN).
5. Set up alerts to stay current on newly-published articles (SmartCILP, SSRN, Wexis alerts, etc.)
Some researches find it useful to conceptualize their research as horizontal vs. vertical. Horizontal resources (such as treatises, books, specialized encylopedia articles, and a literature review in a journal article), might give an overview or background understanding for a topic. Other resources, like a detailed journal article, might delve deeply into the details of a topic. Vertical research can also be conducted for a case or statute. For example, vertical research for a statute might involve looking at it's statutory and legislative history, as well as perhaps any newspaper articles about the statute. Vertical research for a case might involve looking at lower court decisions as well as motions and other docket filings.