With udev / systemd version 241 and similar, as root:
udevadm control --log-priority=debug
Or to make it permanent, again as root:
# edit the log level as described in "man udev.conf"
systemctl restart systemd-udevd
PS: the most frequent yet IMHO wrong answer looks like:
udevadm -d test /devices/where/is/my/device |& less
... but this has a number of issues. The main ones:
where/is/my/device ? Tedious, complicated and error-prone.
Comparing old answers to recent udev version 241 output,
udevadm test seems to show less information that it used to.
udevadm -d test is only a simulation! Every time it warns:
This program is for debugging only, it does not run any program specified by a RUN key. It may show incorrect results, because some values may be different, or not available at a simulation run.
udevadm test is for developing a new rule, it's not for troubleshooting broken, missing or overridden rules.