You get instant insight into a person when you become aware of where their attention is fixed. This is what they are putting their attention on and what they are thinking about.
The is no X-file phenomena, this is something you can observe every day, unless you are medicated so heavily that your senses are dulled.
Your attention is drawn to what the person is putting their attention on.
The person who is completely self-assured can wear jeans in a boardroom and will only attract attention to the topic he is presenting. If his attention is only on his topic and helping his audience, the audience will be captivated by his presentation, not his appearance.
Another example, a competent musician has her attention on the music or on the audience, not on herself or on her instrument:
I was coaching a violinist today, who is preparing to perform before a group of 50 people after not performing for twenty years. She asked me to critique her playing and I was delighted to help out.
She was amazing! Her bowing seemed effortless and her fingering was sure for the first half of the piece. I was totally captured by the sound of the music echoing in her spacious living room.
Shortly after that, I became aware of her anxiety, although her playing hadn't faltered yet. Soon, she came to a difficult passage which taxed her current level of skill and she started making mistakes. Once through those passages, she was off to a strong finish and the music soared through her house again.
We discussed what happened and I mentioned how I was able to anticipate her problem by the way her attention switched from creating an effect with the music to the mechanics of fingering.
She agreed and decided to eliminate the difficult passage for this performance as she is doing it only as a favor to a friend. I am sure she will wow her audience, because her level of skill is such that she creates an emotional effect when she plays, as long as she stays with material that does not task her current level of ability.
You can apply this to your life so you know how much practice you need to perform, or to make a presentation, or to entertain an audience.
To make a professional appearance, you need to prepare until you no longer have attention on yourself or your actions and can focus on your audience and their reactions.
Self-consciousness is actually hard to miss. You will be looking at someone and your attention will be drawn to the fact that they are wearing a wig, for instance. When this happens, it will be because they are very conscious of wearing a wig. Someone who feels at home in a wig doesn't generate that kind of attention.
The same thing is true of someone with attention on a body part, or on a zit, or on the clothes they are wearing. You will look at this person and your attention will be instantly drawn to their body, or the zit or their clothes. When someone is self-conscious about themselves, they are silently screaming, "Please don't notice my body, my zits, my ill-fitting suit", whatever your attention is being drawn to.
On the other hand, a person can be showing off their clothes, or their body, or their hair. You have seen this when someone walks into a room and the first thing you notice is something about their appearance.
This is not an accident. They have a lot of attention on creating that effect. If you have ever seen a top model in a public place, it seems like there is a spotlight playing on them. Your attention will be instantly drawn to them, even if you have no idea who they are. They are "being" a stunning beauty, a real-life Barbie Doll.
There are many other examples of this phenomena, but I'm sure you have the idea by now.
Amuse yourself by noticing people that catch your eye. See what they are putting their attention on.