I numerous places its mentioned that cadets are trained to watch thier surrondings and read body language in other people, while learning to display strong body language.
Does anyone here knows what types of manuals or books they use? I would really like to learn the observational skills of a police officer (without being one :D)
I imagine its very much just something you learn/pick-up while in training and on the ground and not something you can learn from a book (although a colleague did once have an amusing mild descent into madness after reading a book on body-language, she said she couldn’t watch someone cross their legs for example without reading all sorts of subtle meanings and hidden signals into it). 
Also the uniform and equipment is a large factor, they are designed to send specific messages both to the person wearing it and those observing it. Someone will interact with Constable Bob the Police Officer in uniform differently than Regular Bob off-duty cop in civvies.
Sorry Standarduser, I think you’re just going to have to sign up and go through the academy to learn the mad skillz of a cop.
You can google “law enforcement textbooks” to see what is available in that area. A lot of reading body language of perps comes with experience. Nervousness, evasive answers, and avoiding eye contact are some of the most obvious. Someone who is making the decision to run will often shift their weight from foot to foot and tense up right before they rabbit, for example. Addicts who have just scored usually practically run to get someplace where they can use. This is where a good FTO (field training officer) can really make a difference.
Projecting authority and dominance though body language is taught as well. Stance, posture, gaze, and walking like you own the place are key to this. Then there are specific skills like how to stand when interviewing suspects. You’ll notice most officers don’t stand face-to-face but will position their gun side a bit away from the perp. If two officers are interviewing one suspect, they will position themselves and the suspect in a equilateral triangle. This forces the suspect to turn from one officer to the other as the conversation develops and questions are asked, and serves to keep the suspect off balance.
not just that as much as confidence in such situations. As an ex-EMT I have on several occasions come across medical emergencies and when I move to intervene…the crowd just magically parts even 20 years later. You look act you know what you are doing so people assume you are better equipped to handle it.
In the case of cops, they are involved in lots of little scuffles and large ones cuffing people who will not go quietly, a couple dozen of those later, you are pretty confident that any such scuffle will come out in your favor and it comes across in your demeanor. Just like a mechanic who has fixed hundreds of cars, he does not wonder if he can fix it, only how long it might take or if he has the parts on hand.
Cops have a particular was of standing, it’s hard to describe. It’s not aggressive, but nor is it submissive. Not threatening, yet clearly coiled to spring, in a way. They stand a lot, so they learn how to do it better. They know to spread their feet slightly, remain still, not shift weight around unless needed, etc
I worked in a bar, back when yuppies and cocaine were all the rage, and the owner made a handful of staff aware that 2 undercover cops would be in the bar, Fridays for a few weeks. Between us we would play ‘spot the coppers!’, (all in fun, I promise, we respected them and the job they were undertaking!) between us. Before they left they would come to the bar and leave their card for the owner, I guess to prove they were there, maybe get refunded their bar tabs, etc.
I was much better at it than the boys were. Because they were all about haircuts, shoes, size, etc. I could pick them out alright, but it took some close examination on my part to figure out how I was doing it. I realized it was largely how they stood around, for me, that made the stand out form the crowd. They never quite relaxed enough for it to tell in their posture, I think.