A law review/journal article or note must present original research. If another article has already been written on your topic with the same view you wish to propose, your topic has been preempted.
If you find any articles on your topic, but you will address the topic from a new perspective or provide a solution differing from that in the existing articles, you have not necessarily been preempted.
After you have chosen an article topic but before you begin your actual research, check for preempting articles. Also, check again before submitting your paper for publication.
Cast a wide net for articles, working papers, government publications, books, conferences, blogs, interest groups, and other sources which may discuss your topic. Also check for guidelines from the journal or review where you hope to be published.
QuickSearch will search millions of articles in many thousands of scholarly journals---all in one search!
Google Scholar may lead you to articles, conferences, and other publications in databases like SSRN which do not search nearly as well, but have working papers and other resources on your topic.
Search the federal government database of publications, but beware that there are more out there than this searches.
This is but one of several search engines for blogs. Of course, you must assess credibility of the blogger, but sometimes the writer may respond to articles, books, and other good sources for your topic.
LegalTrac is more comprehensive than either of the law journal databases in Westlaw or LexisNexis.