Truth be told, you don’t. Unless you happen to know when and how they pray, as Slate explains, it’s pretty darned tricky:
For the most part, you can’t tell a Shiite or a Sunni by how they look, talk, or dress. There are Shiite and Sunni regions and neighborhoods in Iraq, and members of small communities may know the religious affiliations of their neighbors.
This piece reminded me eerily of the Tutsi/Hutu divide in Rwanda.
DarrenBarefoot.com 
…or Catholic/protestant. Take your pick.
Yea…hi. I really dont know what I am.
I thought one believed that the spiritual leader must be genetically related to the brother-in-law of Muhammad while the other thought otherwise. (I can’t remember which believed what…)
it’s actually nothing like the Hutu’s and the Tutsi’s. Their feud is based on a racial differentiation that was set forth by the Belgian imperialists that had conquered that area. They set one race above the other because of the lightness of their skin and how their nose was set. Basically the more you looked like a white european, the less you were persecuted and the higher you were allowed to rise. In the case of the Suuni and Shi’a, their feud is based off of their religion and who should lead it.