Many people I know can read body language without knowing how
they do it. Some are aware
of this, while others -- well, they can merely observe their reactions
to changes in other people's body languages.
Most such people are regarded as "excellent judges of character", though this isn't always a requirement (I have only encountered 2 people who are quite that body-language-blind, but are still good at evaluating people).
Out of all these people, only a handful have actually admitted this ability to others beside their spouses. When asked, most replied that they are aware how uncomfortable broaching this subject is to other people. Some other replies were based on paranoia, while still others never even entertained the idea of telling anyone.
I've yet to meet a person capable of modifying his/her body language at will in a perfect manner. All I have heard are second-hand rumors are best. I myself have dabbled in this, with some interesting results:
People who are totally body-language-blind are not really affected, unless they've gone through self-training (reading books on the subject is the most common method). Then they are very susceptible to deceit.
People who are unaware of the signals, and of their ability to read them will respond to some of the faked signals, while others are ignored or make them a little puzzled or bothered. This might be due to imperfect emulation of the signals while I'm trying to fake them. Unknown.
People who are very aware of their abilities, and have used/honed them over the years will recognize most (but not all, depending on the amount of faked signals and the subject) of the signals as fake, but only very few were able to penetrate the facade and interpret the real data. Note: This totally creeps me out. And I wish I could do it too.
In my personal experience, modifying your body language is very uncomfortable (like wearing a tight suit), and tends to lend a certain sliminess to the way people perceive you. I'm probably not creating a believable whole out of all the pieces.