What I want to talk about in brief is what it means to listen to your body, and how you can do it relatively reliably.
What is listening to your body?
In short, it is the practice of self-awareness where you take note of how you feel (output) when doing certain things or changing the fuel/stimulus of your body (input).
The inputs can be things like experimenting with different diets, e.g. slow-carb, low-carb, zone, paleo, etc. and the outputs, how you listen to your body, could be noticing responses like less aches and pains, more energy to perform activities, weight loss, different thought patterns, better sporting performance, etc.
A parkour example, to illustrate a slightly different approach, is when warming up for a solo training session. The practitioner will assess her body and how muscles, joints, and mental aspects are operating. Am I sore anywhere? Do I feel up for a challenge or is there a lazy kind of feeling about myself? Then using this information, she will likely decide to train in a way which rests those areas of her body that are sore and allows her to receive the mental stimulus appropriate to the day.
How can you do it?
It's quite easy to copy this approach in a general way. I think most people do unconsciously. However, to add more value to the approach, getting better results from your self-experimentation, it is useful to be a little more rigorous in your method.
One way is to record your inputs and outputs with a journal. A simple spreadsheet with three columns would suffice. One for the input - what are you trying? One for the output - what effect(s) did you notice? And one for any comments or recommendations to yourself for further reflection and action.
It is important to note that some trials should take place over a number of days, weeks, or years. Careful research is also necessary so that you know what to expect with your trials. And finally, it is a great idea to find others who have experimented in a similar way so that you can discuss and collaborate for further added value. Getting others involved, sharing anything learned, that makes all the difference sometimes - that's why I write a lot of what I write, to share the joy!
A final note is that you don't have to wait to experiment with inputs and outputs. You are already immersed in a lifestyle which is producing outputs. See if you can list all of the aches, pains, and annoying things about your body. These are your default outputs and the goal is to improve on these. Sometimes our bodies have been screaming at us for years and we have not noticed!
So there it is, I hope this has cleared up a bit of confusion or brought some clarity to your thinking. Now go forth into the world and listen to your body!