A trademark identifies goods or services from a particular source, or of a defined quality. If a trademark owner would like to use its trademark as its domain name, the owner must apply for a domain name registration with an ICANN-accredited registrar.
I often asked my self that too.If there could be something like a trademark search ingine where you can type in the domain and you get a "go" or "nogo" answer.Just stay away from words like facebook,google and anything that you know is a bussines.And also words like android,ipad,iphone and stuff like that.If you post the name here maybe we can help you faster.
I would recommend checking with the USPTO as they are the definitive source for US-based trademarks. Their search options are actually pretty good. Better than I would have expected from the government http://tess2.uspto.gov/bin/gate.exe?f=tess&state=4810:w2582t.1.1