If you used the --enable-statedir=<STATEDIR> configure option, that will be the
location that holds your logs. If not, the default for STATEDIR will be
<PREFIX>/var/run/watchman, or for older versions of watchman, the logs may be
placed in <TMPDIR>/.watchman.<USER>.log.

Source

"--enable-statedir=#{var}/run/watchman"

Source

Type brew config and look for the HOMEBREW_PREFIX config value. Now we know it’s located at:

/usr/local/var/run/watchman/$USER-state/log