You can't. Or at least, it's very hard to, according to the Congressional Research Service's report on homegrown terrorism:
Scholars and law enforcement officials have noted that no workable general profile of domestic violent jihadists exists. According to the NYPD’s Intelligence Division, there is no effective profile to predict exactly who will radicalize. Another study found only broad trends among domestic jihadist terrorists, specifically that they are overwhelmingly male and about two-thirds of them are younger than 30 years old. As the above discussion may suggest, generalizing about the individuals involved is problematic.
Indeed, there does not appear to be a common thread connecting the U.S. Army psychiatristMajor Nidal Hasan with the Caucasian convert, Daniel Patrick Boyd; the Afghan immigrant Najibullah Zazi with Carlos Bledsoe, an African American of a happy childhood who converted to Islam and renamed himself Abdulhakim Muhammed; David Headley, who was born Daood Gilani to a successful Pakistani immigrant father and American mother, with Talib Islam, who was born Michael Finton and raised in multiple foster homes; or the educated pharmacist Tarek Mehanna, with the Somali American from Minneapolis Shirwa Ahmed, who traveled to the land of his birth and became the first U.S. citizen suicide bomber. The plots and attacks drew in first-and second-generation Muslim-American immigrants and native-born Americans who converted to the faith. Some included individuals acting alone, while others had multiple co-conspirators. Some plots were aspirational. Others appear to have been pushed along by government informants or undercover agents, and still others were serious and calculating until uncovered by intelligence and/or law enforcement officials.
Here's another problem. The most successful homegrown terror plots, at least in the sense of actually resulting in someone getting killed, didn't involve bombs or weapons of mass destruction, or even multiple conspirators. Major Hasan's attack on Ft. Hood that killed 13 people and Mohammed shooting two army recruiters, one of whom died, involved someone simply picking up a gun and deciding they wanted to kill people. In Hasan's case there were some warning signs about radicalization that were missed. But in general it's extremely hard, in a country that respects the individual right to bear arms, to prevent someone who wants to get a gun and shoot people from doing just that, no matter what the motivation. It's a very low-cost form of terrorism, and its effect is magnified by the ongoing domestic political dynamic here in the U.S.
Of course, the amount of damage sustained as a result of such attacks is partially dependent on how we react to them. As James Lamond points out today, a recent report on assessing the threat of terrorist groups by Peter Bergen and Bruce Hoffman noted that:
If any attack can succeed in generating significant political and economic fallout, then there is a greater motivation for undertaking these attacks. Alternatively, terrorist attacks that have limited potential to inflict serious casualties or cause disruption become less attractive if Americans display a greater degree of resilience by being better prepared to respond to and recover from these attacks. Since as a practical matter it is impossible to prevent every terrorist attack, the United States should be working in any event to improve the capacity of its political system, along with citizens and communities, to better manage how America deals with such attacks when they occur.
Given the success of Hasan's shooting spree in generating panic, it's no surprise that al-Qaeda propagandist Adam Gadahn called him, somewhat hopefully, a "trailblazer." Whether or not more of these attacks happen depends at least in part on America's ability to -- you might say, "absorb" -- such incidents rather than reward terrorists with the exact kind of fear they were hoping to provoke.
UPDATE: I'm amused by some of the comments on this thread to the effect of, "the common thread is that these terrorists are all Muslim." Congratulations: You've just narrowed it down to 1.5 billion people.
