I think the whole idea is: you can't always get what you want, but if you pray sometimes you might just find you get what you need - seriously plagiarising The Stones for that - but I think at least from what I understand, that is the point of prayer.
It's not a miracle or and altering of the laws of physics and reality, it's just a little divine help in a time of need.
But I suspect that this is unhelpful. What I think you want to ask is how I can know for sure which cases are intervened in by God immediately and which cases are the result of God saying "Let there be light" a long time ago, and my answer is that I can't. I am convinced but not certain.
