The USPTO publishes its operating manual for examiners on its website. All of chapter 900 deals with prior art and searches for it, but an overview of the process can be found in these sections:

904: http://www.uspto.gov/web/offices/pac/mpep/documents/0900_904... 904.02: http://www.uspto.gov/web/offices/pac/mpep/documents/0900_904...

The process examiners use for a specific patent is also documented by them in the file wrapper for every application and is available in PAIR: http://portal.uspto.gov/external/portal/pair