Identification , which is lacking from countries with no functioning government, references and cross checking reports and captured intel , Fingerprint matches taken from IED and confiscated weapons< interviews with immigration officials and other organizations,, running them through facial recognition, also cross checking social media and cell phone tracing, as well as possible DNA tests in extreme cases where terrorist ties are suspected.
And by the way , I ain"t dropping this issue since the Security of The United States is an issue and should even concern Everyone in this country since they want us destroyed.You are always welcome to fly over there and help them fight for themselves like we Have for our Country !