"He called on the American public to remain a part of this effort, "to do your part, to remain vigilant and ever-watchful for suspicious activity and behavior, and to report that activity."
This is one of the important ways of discovering them. The intel services, police, etc. can't be everywhere.
In recent years a lot of tobacco shops have opened up in the Seattle area that are owned by Middle Eastern men. There may be an innocent explanation for this, but if I were the FBI I'd be keeping an eye on them. Also, I went into a 7-11 recently, late at night, and the Black and/or Middle Eastern clerk was listening to what sounded like an angry rant in a foreign language on the radio. When he noticed that I was noticing, he turned off the radio.