The idea of a preemption check is simply to check to see if your topic has been covered before. Sometimes paper authors get far into their research before discovering that their topic, thesis, and focus was sufficiently covered in others' works.
Find papers on your subject? Good! Now you can refocus your paper and you now have an excellent source. Consider:
Run a preemption check early and you will not only save yourself time and disappointment, but will certainly end up with a better focus and scholarship. "Run a preemption check" simply means searching for articles on the same topic you intend to write about - but be sure to look further than Westlaw and Lexis as other databases may have wider coverage dates and include more non-legal journals.