But how does a Shiite look at another mohammedan and determine that he is a Sunni, and vice versa? What do the members of the two different groups actually do in their daily lives that is different from the behavior of the adherents in the other group?
Shia really like calling their kids Hussein and Ali. If you ever see a Abu-Bakr, Omar, Muwaiyah or Yazid, he's almost certainly a Sunni.
Shia also believe in respecting the graves of Mo's Companions, and pray at their graves out of respect and often requests for their spiritual intercession. Sunnis view this as grave-worshiping and therefore shirk (polytheism) and equivalent to apostasy. Sunnis would actually prefer to destroy all the graves, bodies and monuments that encourage shirk.
This is a BIG deal for Muslims, because apostasy carries a death sentence. Therefore this practice marks all Shia for death in the eyes of some Sunnis. It's roughly comparable to Catholic veneration of saints and beliefs in Intercession, as opposed to Protestant minimalism.
Although obviously it's not as big of a deal, as Christianity doesn't demand death for Christians who may or may not have deviated.
Some Shia also engage in self-flagellation. Shia have also historically used mutah (temporary marriages) to justify Islamically what is basically prostitution.
The answer is distinctive head dress. But the Sunni also attack Shia market places and take their chances ( no Sunni would ever go to a Shia market). Even the women wear color differences. Let me assure you that religious distraction is averted gaze in the leftist press. Educate yourself and you will see a different picture. It is important to your family.