The best thing is if the person herself is in tune with her body and she feels what is going on with it. But even if she doesn't, the facilitator will be able to pick up much information.
A person's external body language speaks volumes. By observing somebody's posture, eye movements, breathing and skin color, you can gain information about what she is doing in her mind. You can get the same information by listening to the qualities of her tone of voice. And this is without being psychic, just by looking and listening. If you add a bit of ESP to it, it just gets much easier.
Body language is a big subject and to master the reading of it to perfection requires a good deal of training and experience. Here we will just present some of the most practical things you can notice.
In the absence of knowing what different body movements mean, you can simply notice when there is a change in the client's body. For example, if she is sitting perfectly still and suddenly starts twitching or moving her eyes - that is a change. Or if she changes her rate of breathing - that is a change.
Let's say you have asked the client to close her eyes and move back to a past incident. At first she is just sitting still, her eyes not moving. But suddenly you can see her eyes moving behind her eyelids and her head is jerking a little bit, and maybe her breathing gets faster. Well, that most likely means that she found an incident. Interestingly, she might not herself have noticed that. Subconsciously she has the incident right there, but consciously she might not have acknowledged it. So, if she doesn't start speaking by herself you can say "What's that?", or "What do you see?"
If we are dealing with an incident, the body will often show what is in it, or how she relates to it. If her eyeballs are moving, it means that there is something to look at. If she suddenly breathes pantingly, it means something exciting or stressful is going on.
If the client is leaning forward, she is probably into the incident, involved directly in the action. If she is leaning back, she is probably seeing it from a distance.
You can not be sure, just from a specific body motion, what is going on. But it gives you a very good idea. Particularly when you notice the changes in response to your directions. If you ask "Is there another viewpoint in the incident?" and she suddenly leans back, that probably means that it is an external viewpoint, seeing things from a distance.
There are a whole set of signals that tell you what kind of perceptions the person is accessing. To make things simple, we can divide body perceptions into Visual, Auditory, and Kinesthetic. I.e. pictures, sounds, and feelings. That is not only what the person mostly takes in in the present, it is also mostly what she has stored in her mind. Most of what you find in the mind is made out of some sort of combination of pictures, sounds, and feelings. When the person accesses an item in her mind her body will tend to reflect what kind of information it is, if it predominantly has pictures, sounds, or feelings.
Feeling has a low frequency, it is slow and deep. Auditory has a higher frequency, it is faster, more mobile. Visual is high frequency, fast and changeable. These qualities show in various ways in the body.
A person will breathe shallowly and high in the chest when accessing visual information. She will breathe deeply and slowly when accessing kinesthetic information. And somewhere in between when accessing auditory information.
When accessing visual information, the person will tend to look up, straighten up her body, and make gestures into space. When accessing auditory, she will tend to look to the side, maybe slant her head and cross her arms. When accessing kinesthetic, she will tend to look down and slump over.
The person will speak faster and more high-pitch when accessing visual information than when accessing auditory information. When accessing kinesthetic she will speak with a lower, slower tone of voice.
You can be more or less of a specialist in interpreting all these body reactions. You don't really need to know them all that well to do basic processing. As long as you notice that there are changes and reactions, and you notice when the client is looking happy and content, and when she isn't, then you can do just fine in your sessions.
But if you know body language really well you can do magic with it. People will swear that you are reading their minds, when really you are just looking at them.