XnView Software

I would like to know the categories a file is assigned to.
I know about two ways:

1. By context menu / category
2. By viewing the category panel

Both ways I have to check ALL existing categories (and to expand
the sub-trees) to manually identify the bold/ checked ones.
Which is anything but practical if you have more than about 100
categories defined.

Isn't there a practical way?
Somewhere in the "properties" etc.?
Showing only the assigned categories?

The Category Sets panel should display what you are looking for.

... and there the system category "Assigned"...

Yes, you are right.
That was something I wasn't thinking of.