I'm not a Patent Attorney, but have used thier services a few times.
I've been told that as long as the word is in the dictionary, that means in common and can not be trademarked. If the other hand, the word is unique, and not in the dictionary than it's best to stay away from it.
You can also do a search here http://www.uspto.gov/trademarks/index.jsp
This is just my experience, and it's best to checked out with a Patent Attorney (~ $400)