On OSX Homebrew installation the log can be found at:
Latest Homebrew:
/opt/homebrew/var/log/postgres.log
or older:
/usr/local/var/log/postgres.log
or for older version of postgres (< 9.6)
/usr/local/var/postgres/server.log
Bonus – check if PostgreSQL is running using Homebrew:
brew services info --all