How Can ISIS Tell Who’s Sunni or Shiite?

I was reading about ISIS capturing a bunch of air force cadets who were fleeing their base outside Takrit, and letting the Sunni ones go, but keeping (killing?) the Shiite ones.  And I was wondering how the hell they know who is Sunni and who is Shiite in a mixed setting like that.

I found a Slate article from 2006 by Daniel Engber that says it’s not that easy to tell.  Some surnames are usually one or the other, but there’s been a lot of inter-marriage, so you can’t be sure.  He says some first names are traditionally one or the other — Ali, Hassan,  and Hussein are Shiite, while Omar, Abu Bakr, and Yazid are Sunni.  But if a guy’s named Mohammed, you got nothing.