There's really no surefire way of being able to check if a word is being used as a trademark or not, although some online sites can at least give clues of such like what eddy posted. Two attorneys I know love to google around for such, just so they have some hints to work on.
There are companies that specialize in trademark searches. They check a lot of various sources and look for possible infringement issues. Trademark infringement does not need to be the exact phrase and it does not need to be registered with the USPTO. A good search will probably run a few hundred dollars and still won't offer 100% protection but will give you a good idea on whether or not you should proceed. As suggested, use google to see if you can find anyone else using the term or word in the same classification you want. If you can find prior usage, I would then pay for the trademark search. The registered trademark process takes about six years from filing to become uncontestable. You don't want to spend 5 years developing a trademark only to have it successfully challenged and loss the rights to the mark.