“After my death my nation will be divided into 70 sects, all will go to hell but one.” This is what the prophet Mohammad foretold, and this is what happened as far as I know, but surely no one can tell which sect will go to heaven concerning the second part of the prophecy, although each sect proclaims that they’re the ones. The two major sects in Islam are the Sunni sect and the Shiite sect. They are called Sunnis because they follow the Sunna of the Prophet Mohammad, which means his sayings and doings. And the others are called Shiites because they are the followers or supporters of Ali ibin abi Talib, the Prophet’s Cousin and son-in-law, so they’re called Shi’at Ali, or the partisans of Ali. You can tell if a person is a Sunni or Shiite based on names, appearance, slight differences in prayers, places of prayers, and certain special rituals and celebrations.
- Names: In spite of the sect, Muslims around the world use some common names to give to their children, such as Mohammad (after the prophet) or Ahmad, which is derived from the name Mohammad. Yet there are some names that have a sectarian marker and, if used, determine almost for sure whether a person is a Sunni or a Shiite. Such names include Ali, Hassan, Hussein and Zahra for Shiites and Omar, Othman for Sunnis. As for Muslim women, name differences between Shiites and Sunnis are slight, although I can say that Shiites tend to use the name Zahraa for their daughters.
- Appearance: Those who shave their heads and mustache, with a small cap on their heads are Sunni religious people. Others with higher rank “sheikh” have a turban which consists of a long piece of fabric wrapped around a small red cap. The Shiite clerks may have a black turban wrapped around the head, which tells that this clerk is from the bloodline of the prophet and called Sayyid. Otherwise a clerk wears a white turban to indicate that he is not from the same bloodline of the prophet.
As for secular Shiites (they are mainly affected by the Persian style), more than 60% of the Shiite Muslims of the world are located in Iran (formerly Persia) and wear no tie, a shirt with no color, and some tend to grow their beards a little or trim it without shaving. As for Shiite youths they have tattoos on their arms, such as names of Ali or Hussein his son, or the sword of Ali, Thu alfiqar (“sword with a split head”). They even have necklaces with the sword. In their homes, Sunnis have verses from the Quran, while Shiites have portraits of Hussein picturing how he was killed, which is an Arabian knight on a horse leaning to the side with a spear in his chest. Also they have portraits of the sword Thu alfiqar, and the name of Ali and Hussein in frames. The same goes in their cars and neighborhoods.
As for women, it's sometimes hard to tell the difference between Sunni women and Shiite women based on appearance, because of the fanaticism in the implementation of the Islamic rules (or in other words, the ways of the prophet). For instance, Saudi Arabia is a Sunni Muslim country where women are forced to wear something called "shador" which is a black dress that covers the whole body from top to toes. On the other side Iran is a Shiite Muslim country where women are also forced to wear the "shador" whether they like it or not, exactly like in Saudi Arabia.
Nevertheless, in some moderate Islamic countries, like in Malaysia (a Sunni Muslim country), women tend to wear a veil that covers the head, neck and ears, and they wear pants and long shirts not to reveal their beauties, or they can also wear long dresses not revealing their flesh. As for moderate Shiites, they also get dressed in the same way, but they try to differentiate themselves in a certain way just to recognize each other. For instance, in Lebanon moderate Shiite women wear their veil in a different way where they put a pin on the left side which would make it look different and recognizable.
- Difference in prayers: Since the days of the prophet, when calling the faithful for prayer, Muslims invoke God and the prophet Mohammad, but Shiites added Ali. Muslims (Sunnis and Shiites) use a small rug or carpet in their prayers. They kneel, bend and touch their forehead to the ground, but Shiites unlike Sunnis touch their heads to a small stone called turba, from the holy city of Najaf. When they pray, Sunnis keep their arms one folded over the other just below the rib cage, while Shiites keep their arms down. Additionally, there are differences in times of the prayer, where Shiites are minutes behind Sunnis, perhaps just to emphasize the difference. That difference goes also to the fasting time during the fasting month of Ramadan, where they break their fasting at slightly different timing and celebrate the end of Ramadan (the ‘Id) a day or two apart.
As for women, whether Sunni or Shiite, Islam favored men over women; women according to the Quran are not equal to men, so they pray at home and not with men in mosques. Men and women are not even allowed to mix together in public places or schools on any occasion.
Sunnis pray in mosques, and those mosques have domes and minarets, while Shiites pray and worship most often in Husseiniya (no domes or minarets), which functions like a mosque and a community center and a place where they share condolences during the memory of Hussein’s death, son of Ali.
- Rituals: Both Sunnis and Shiites celebrate the birth of their prophet, and the Ezra day, but only the Shiites have the Ashura day, where they gloomily commemorate the killing of Hussein the son of Ali by Sunnis, one reason why these two sects have hatred for each other. They share condolences for ten days in Husseiniya (you can see that the word is derived from the name Hussein). For ten days, Shiite men are dressed black, and they don’t shave their beards. Secular women wear no make-up or perfume, and if they have a veil, it must be black. On the tenth day, they march on the streets and beat their chests in mourning. Some whip themselves and cut their heads and hit on it to bleed, because by this they show how much they regret not preventing that from happening. By Sunnis, this is seen as something unpleasant, and uncivilized.
I’m sure one can find more differences between those two Islamic sects. I hope my experience derived from living among them and having friends from both sects will help you to tell the difference if you need to.