It is absolutely necessary to find out if there is anyone using your mark prior to your use. A clearance search is often performed through proprietary services, which often search not only state and federal registrations but also common law trademarks. Typically, a better service costs more; but depending on your situation a simple Google search may suffice.
Typically, a clearance search provides lists of all current state, federal, common law trademarks that are identical or similar to your mark. Unless your mark is very unusual, you will face a number of identical and similar trademarks in use. Even if there is an identical trademark registered and used, it does not necessarily mean that you can't use the mark; however, it is also true that even if there is no identical trademark, you might not be able to use the mark. At the end it all comes down to whether your use of the mark would infringe other trademark owner's rights.
Are you worried about (or disappointed with) your clearance search? Search trademark-infringement