How can I tell which homebrew formulae are upgradable?

I know when I brew update, it lists all ==> Updated Formulae, but when I've updated several times without running brew upgrade, how do I get a list of all apps that could be upgraded?

Per homebrew code on github, this shows all apps that can be updated:

brew outdated

brew help does not list the command, but it is documented in man brew.

Since Homebrew 2.6.0 released in 2020.12.01, brew outdated behaves a little differently.

# update package repo
brew update

# list outdated pkgs, both formula and cask
brew outdated
# list formula only, the old behavior of 
#  `brew outdated` before brew 2.6.0
brew outdated --formula
# list cask only
brew outdated --cask

In Homebrew's jargon, a package is called a "Formula". Homebrew is not only manages TUI apps, it's also capable of managing GUI apps. A package for a GUI app is known as "Cask".

Before Homebrew 2.6.0, brew outdated only list formulae. Since 2.6.0 brew outdated both of formulae and casks.