Check this site for more info:
http://www.acf.hhs.gov/trafficking/index.html
Some signs:
-Evidence of being controlled
-Evidence of an inability to move or leave job
-Bruises or other signs of battering
-Fear or depression
-Non-English speaking
-Recently brought to this country from Eastern Europe, Asia, Latin America, Canada, Africa or India
-Lack of passport, immigration or identification documentation