The following site is an excellent tool to identify Giant Hogweed (one of the best sites I've seen). There are comparison charts of similar looking plants as well. Giant hogweed identification:
http://www.dec.ny.gov/animals/72766.html
Comparison photos of Queen Anne's Lace and Giant Hogweed:
http://www.hort.uconn.edu/cipwg/hogweedLookalikes/HogweedLookAlike_WEB/QAL.htm
The following link provides a map of north America, shaded portions indicate where Giant Hogweed has been confirmed: http://plants.usda.gov/java/profile?symbol=hema17
This link shows what contact with any portion of the plant can do to a human: http://www.hometalk.com/1706763/hogweed
