Most people can tell you who the potential jihadists are, especially the ones in Europe and the USA. You can point them out in a group: they are immigrants, or more often, the children of immigrants, who came from the Middle East or North Africa. They often converse among one another in Arabic. Many want to join the Islamic State and other terror groups in Syria, or have gone and since returned. They are mostly men, usually around age 20 or less, and have grown up feeling alienated from the societies in which they live.