For instance, root partition is full so I don't need to know about /ABC/XYZ when /ABC/XYZ is a separate mount point. (But /ABC isnt).
Can I run a du command or similar and just look at contents effecting the space on that mount point (/)?
Why not just use df / ? Maybe with a -k or -H option depending on what units you want.
The du command supports the -x flag to prevent you decenting into sub-mounted filesystems.
I usually do:-
cd /mount/point
du -kx . | sort -n
This will list the directories in ascending content volume. The value is cumulative, so you might have to change into the various sub-directories and work your way down.
I hope that this helps,
Robin