how then I can detect if focus finished successfully or not?
Zoom you camera to telephoto.
Point it at something 150mm away.
After it has tried to focus, focus_busy will be zero.
It has finished trying.
BUT, it can never focus at 150mm for optical reasons.
In very dark conditions I guess it may have similar problems.
I tried monitoring focus_busy a few days ago on an A460, did not seem to work.