How to get a list of all owners of files in a directory

You can use find to print the user (owner) and group and then extract the uniq combinations e.g.

$ sudo find /var -printf '%u:%g\n' | sort -t: -u
_apt:root
avahi-autoipd:avahi-autoipd
clamav:adm
clamav:clamav
colord:colord
daemon:daemon
lightdm:lightdm
lp:lp
man:root
root:adm
root:crontab
root:lp
root:mail
root:mlocate
root:root
root:shadow
root:staff
root:syslog
root:utmp
root:whoopsie
speech-dispatcher:root
statd:nogroup
steeldriver:crontab
steeldriver:lightdm
steeldriver:steeldriver
syslog:adm
systemd-timesync:systemd-timesync
testuser:crontab

stat -c %U * 

will list owners of all files.

This can be sorted and duplicates removed by piping it into sort -u:

stat -c %U * | sort -u

As pointed out by steeldriver, this is not recursive. I missed that this was asked for. It can be made recursive by enabling globstar:

shopt -s globstar
stat -c %U **/* | sort -u

Alltogether, steeldriver's answer is probably better and should be the accepted answer here :)

You may find it more efficient to directly search for the files not owned by the user ...

find /directory ! -user username -printf "%u %p\n"