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?

What does Homebrew upgrade do?

brew update and upgrade brew update updates the above downloaded git repository with the latest code from GitHub. brew upgrade updates the actual packages to match the versions in the updated local git repository.

How long does it take to update Homebrew?

From 20 minutes to an hour. The devs are aware.

What does brew list do?

brew provides commands for getting common types of information out of the system. brew list shows installed formulae. brew deps foo shows the dependencies that foo needs. Additional commands, including external commands, can of course be written to provide more detailed information.

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