The purpose of preemption checking is to ensure that your thesis is original. The preemption check should be performed prior to any serious writing.
There are two types of preemption:
Type 1: As a matter of law
Hypo: Suppose your thesis argues that law schools typically loan their law students too much money for their earning potential; and, in doing so, law schools violate the Family Educational Rights and Privacy Act of 1974.
However, after researching using the methods taught to you by those at the Rinn Law Library, you discover that a court of competent jurisdiction has already ruled on that exact issue for the same exact reasons you set forth.
Have you been preempted?
Yes, as a matter of law.
See Slovinec v. DePaul Univ., 332 F.3d 1068 (7th Cir. 2003).
Type 2: By author
Hypo: Suppose your thesis argues that law schools typically loan their law students too much money for their earning potential; and, in doing so, law schools violate the Family Educational Rights and Privacy Act of 1974.
However, after researching using the methods taught to you by those at the Rinn Law Library, you discover that there is a law review article written on that exact issue for the same exact reasons you set forth.
Have you been preempted? Yes, by author.
Basically, you are to: 1) methodically identify all the different sources where you could reasonably suspect your thesis can be found; 2) meticulously search said sources using varying research strategies; 3) record your results on a research log; and, 4) set alerts. Note also that a thorough preemption check should be conducted before any significant writing but is in and of itself a continuing process and has to monitored throughout the writing process.
LexisNexis has a great video on how to conduct a preemption check (starts at 29:25).
CALI also has a preemption check lesson where you can develop your skills in the preemption process.
While a lot of sources contain overlap with each other, certain sources are known for publishing certain types of materials. Therefore, knowing where to search when conducting a preemption check turns largely on the topic you intend to publish. With that said, this is a list of some of the sources worth checking:
1. For a historical archive of primary and secondary sources in PDF, see HeinOnline.
2. For interdisciplinary publications, see JSTOR.
3. For new and unpublished working papers, see SSRN.
4. For a broader and more general search, see Goggle Scholar.
5. For new law journal publications, see Current Index to Legal Periodicals.