Hi, while using Trace Bitmap on a PNG, how do we know that a trace is complete and the output is as good as it can be ?

Also, what is the SIOX option which is unticked by default ?

Thanks.

Not sure about the first question, but for the second:
http://www.siox.org
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Trace-SIOX.html

The status bar updates while a trace is taking place; the resultant path is automatically selected, so is listed on the status bar after completion; the "Apply" button in the dialog remains in its pressed state while tracing, and only becomes available again once the trace is complete.

As for the output being as good as it can be - all you can do is try it, and experiment with changing some of the parameters if you feel that something better can be produced. "Best" and "Better" are subjective terms: one person may want a path with a small number of nodes, smoothing out inconsistencies in the lines, while another may want a huge number of nodes to reproduce the shapes as accurately as possible.

I covered the Trace Bitmap dialog in parts 19 and 20 of my tutorial series in Full Circle Magazine (free download), and more recently in parts 111 and 112 (due out this month): https://inkscape.org/forums/tutorials/inkscape-tutorials-in-full-circle-magazine/

In part 20 I had this to say about the SIOX option:

In my experience, you would be better off separating out the foreground using The GIMP (which has its own, more interactive, implementation of the SIOX algorithm) and then tracing  the result.