How can the Arizona police tell if someone is "reasonably suspicious" of being in the U.S. in "illegal" status?

Usually I answer the questions posed by my readers myself. But in this case I beleive that comedian John Stuart has given the best explanation so far of how Arizona police will know when they should ask someone to provide immigration documentation because there is a reasonable suspicion that they are in the U.S. illegally.

The Daily Show on the Arizona Immigration Law