The documentary you are talking about is called Inside A Cult. You can find more information here: http://channel.nationalgeographic.com/series/inside/4097/Photos#tab-Overview
Brainwashing uses subtle techniques that most of us are unaware of. Take for example the technique call conditioning. When I learned multiplication, I did so by memorizing multiplication tables: 1*1=1, 1*2=2, 1*3=3…. Constantly repeating the same series of numbers or words is a brainwashing technique that is used to place knowledge directly into long-term memory, bypassing short-term memory. All religions use the same technique. Take Christianity for example; visit a Sunday School class of children. They’re all singing “Jesus loves the little children, all the children of the world…”. They all know the song without hesitation. The words of that song have been planted in their long term memory. They may not be old enough to know what the words mean, but the words are there nonetheless. Islam uses the very same technique. If you have ever seen the students in a Madrasah dīniyyah, you will notice them all reading their Qur’an, rocking back and forth in unison. It is the same technique whether it is used for math formula or religious stories.
People are usually not aware that they have been brainwashed, but brainwashed people are easy to spot. One classic sign of a brainwashed person is denial. Ask them if they are brainwashed and they will tell you that there is no possible way. Another easy way to tell if a person has been brainwashed is their use of suggestions that have been implanted during their brainwashing. For example, if someone recites: “The fool says in his heart, "There is no God." They are corrupt, their deeds are vile; there is no one who does good.”, you can bet your bottom dollar that they have been brainwashed.
There are professional organizations that specialize in helping brainwashed people realize they have been brainwashed. These organizations are called exit counseling. Their success rate is high, but their biggest obstacle is to get the patient to come in for treatment. That’s the hardest part because brainwashed people are in self-denial – they deny that they have been brainwashed.