When we do a simple observation of breath (in & out) during meditation, how to know if its natural flow I am observing or forced one?
If your aim is vipassana then what forced the breath? If the breath is forced or not forced that is what is and can just be witnessed mindfuly as it happens. If the breath was seemingly controlled by you, were you in control if you couldn't even tell when you were forcing it or not?
If you just keep watching mindfully then you will see everything is rising and falling constantly while being forced by nobody.
This tripped me up in the beginning. I was told to let it go because if I was controlling it or not, ultimately I wasn't in control anyway.
In whatever way objects come, do not try to change them. Whatever comes is the buddha-dharma, not objects at all. Even if you try to control what comes, it cannot be controlled. -Dogen
If the instruction says "breathe naturally" I think that means "just breathe" and "don't try to hyperventilate".
When I cycle (exercise) my breathing increases to meet the increased demand (and my heart does too) -- don't have to "force" that -- couldn't even stop if I wanted to!
So it's rare that I intentionally "force" a breath -- I think that "force" means "breathe before I need to", or "more than I need to"? -- I do that rarely, only to make a temporary surplus of oxygen, a few seconds before a sprint.
If you're worried, try not breathing?! Then instead of "you" forcing "the body" to breath, the body will force you to breathe! So I let the breath out, relax, pause, and the body takes another breath. And I let it breathe again before it becomes uncomfortable. And so the breathing (at rest, not during exercise) tends to be a little slower (waiting for the body), and deeper/bigger breaths.
That's my experience, perhaps someone can correct me if I'm wrong.
One other thing I've found useful to know of: if I do force the breath then I force it out (exhale), not in. If you only try to breathe in, without breathing out, then the lungs accumulate stale air. Better to breathe out, let go, then the body will breathe in again.
ChrisW's advice to not breath is a really good one. I always use that method in my meditation when I find that I'm 'attacking' the breath or being otherwise willful with it. Another method I'd add is 'waiting for the breath'. Just sit and wait for breathing to happen - like you're sitting on a bench waiting for a bus to come. Both will help you experience natural breathing.