Working papers are now often posted online for discussion and sharing before they are published.
SSRN (the Social Science Research Network) has almost 400,000 abstracts (over 325,000 of them with papers that can be downloaded). It includes unpublished papers (the Working Papers Series) and papers that have been published or accepted for publication (the Accepted Papers Series). You can search the abstracts with keywords. You can also browse by subject.
For more on SSRN, see this guide.
bepress (Berkeley Electronic Press) hosts repositories for many universities and law schools. You can search the collection. Some but not all of the papers are available for download.
The Legal Scholarship Blog posts announcements of calls for papers and upcoming conferences. It also lists many in-house workshops and colloquia in U.S. law schools. If find that a law professor is workshopping a paper on your topic or that another law review is hosting an entire symposium on it, you might be alerted to a preemption problem long before the article is in print.