Once you've established your claim, you need to monitor the topic and any important cases/statutes you will be discussing to make sure that nothing important changes between when you've done your research and when you finish your note. There are several services that make this monitoring easier:
You can never know for sure if someone else is working on the same topic you are, but you can review some articles before they are published.
Potential law review note topics must be throughly reviewed to ensure that they are presenting a novel claim that has not been previously addressed or made obsolete by further legal action. This does not mean that your topic must be completely obscure or entirely novel, just that whatever nuance or angle you wish to approach the topic with must be original. If you discover that your proposed claim has been addressed by someone else or that the case you were basing your argument on has been reversed or superceded, you will need to alter your approach. This review is essential to determining what materials are relevant to your argument and has to be repeated throughout the research and writing process.
Luckily, a thorough review of relevant material is necessary to the writing process anyway, so once your topic passes its preemption check, you will already have a significant portion of your research completed. Be sure to keep good records of the material you review. The best place to start when searching for existing coverage is a standard search of the legal literature - resources for this search are listed below.
Once you are satisfied that your topic is sufficiently novel, you can shift into monitoring mode, to ensure that further developments do not preempt your topic without you knowing about it. There are several services that will provide topical reports and monitor case developments for you, and they are listed in the left hand column.
A preemption check involves thoroughly reviewing existing scholarship to ensure that your proposed topic is an original angle and that any key cases or statutes that your argument is based on are still good law, if relevant.
To review legal scholarship, conduct searches in multiple databases of law review articles. Use several different combinations of keywords to make sure your searches are sweeping up as much relevant material as possible.
You will probably also want to spend some time just googling keywords related to your topic, to get a sense of any commentary that may not be officially published.
For any cases or statutes that your argument depends on, you will want to Shepardize (Lexis) and Keycite (Westlaw) those citations to ensure they are still good law, and set up alerts for each service to notify you of any further changes (see left column for information about setting up alerts).