In the internecine warfare in Iraq, in addition to bombings that target neighborhoods known to be predominantly one sect or another, many of the murders are up close and personal. The news reports “20 Sunnis murdered” etc.
How do the murderers, death squads, etc., know a Shiite from a Sunni? Traditional dress? some linguistic shibboleth?
While there are a few minor personal clues, mostly revolving around slight variations in prayers and praying style, for the most part there is no obvious identifier.
It’s more that everybody knows everyone else’s business. What mosque you go to. For some what crowd you hang out with. Not infrequently ( and sadly increasingly frequently in Iraq ) what neighborhood you live in. What tribe, sub-tribe, clan or family you belong to. That sort of thing.
The same way that Christians tell Catholics and Protestants apart. People in the neighbourhood know who goes to the right place of worship and who goes to that other one.
They let Allah sort it out.
Well, that appears to be the CURRENT procedure, Spezza. 
It has more to do with the fact that they have self segregated themselves. The deathsquads/bombers attack specific neighborhoods when they are not attacking specific people. Sometimes someone unintended gets killed too. Nobody’s perfect.