Sensing the energy in the opponent

It has been a while since I notice how students are powering up when they engage with each other while I am watching from the side without touching them. I am not yet sure how I can tell. Is it their subtle movement, body language or something else? E.g. I can tell if they are pushing from the heel or toes, if they are powering up with a back muscle, at the shoulder, or with the kua. This information is useful in identifying the fixed dot in the opponent's body. It allows me to help the students understand their bodies, and what they are actually doing. They ask me how I can tell, however at this point, I honestly don't know how I know, I just do. It just seems like the energy is running through the body.
Today, I was teaching a student how to kick with heel. The focus was to stretch the back of the leg and not let the energy get stuck on the knee. I was telling the student the difference between the proper and non-proper ones. I realized that I was not using my eyes or any touch to tell if she was stretching or not. The leg might be extended but not necessarily stretched. We did a test with me looking a way to see if I could tell if she was stretching or not. We repeated the test a few times, and somehow I could tell. Was I using some sort of sound? I was not sure, I just felt it in my body somehow. This was written for the sake of recording the incident.
One of the ways to learn to locate the fixed dot is to put some pressure on the opponent and see how he reacts to it, e.g. how he resists it. The same person tends to react the same way, the more you work with a person, the more you are used to his actions and can anticipate what he will do. The more people you work with, the more developed the skill. It eventually helps in dealing with you have never met before.