Question about Iraq sectarian violence

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The Shi'as and the Sunnis are murdering each other (mainly I believe it's Shi'as, a more fundamentalist group, killing the Sunnis - a more "laid back" sect, if I can characterize it that way).

Simple question, and I say this only because I haven't been there and aren't trying to be willfully ignorant, but how do they know who is who? Is it specific physical characteristics? Taller, darker, shorter, lighter? Specific manners of dress or grooming? Beards vs. non-beards? Clothing or headware? Stamps on their ID cards? Language? Neighborhoods where people just know who is who?

There is just so much of it, where bands of Shi'as go through and kill Sunnis (and probably vice-versa) - I was wondering what, if anything, made it obvious that someone was of a particular ethnic group that made it clear they were an appropriate target.