People will tell you a lot if you pay attention. People ask me all the time "how did you know...?" but they told me, either in behavior or mannerisms or whatever. Just be observant and you'll pick up a lot more than people know they're letting on.
Interestingly, your not. A lot of it is learned, rather than instinctive. The only bit you get fucked on is the mental energy side of things.
If you want a primer, the show "Lie to me" is quite good. At least season 1 is. It is a drama, but actually has a good scientific backing, at least in season 1.
The quote I always heard was "Actions speak louder than words". I wonder if they mean something slightly different though.
They way I can see it is your quote "You say a lot when you aren't saying anything" feels very passive, as in the way you carry yourself on the day to day.
Whereas "Actions speak louder than words" is more active (has that word in it alredy) as in how you treat and carry yourself around others.
Both can be seen as a way to observe the social queues that aren't completely verbal to see what kind of person someone is. Just something to ponder I guess.
This isn't even necessarily just true for non-verbal stuff...I've had people ask me how I knew things they had personally told me, to my face, about themselves. Some people don't remember everyone they tell something too and can be suprised later that you know something.