How to see which Mac apps need an update
A Mac has no single place that knows every app's newest version. Each app updates through the way it was installed, so checking them all means looking in three places.
The short answer: App Store apps update in the App Store › Updates. Apps from a website usually have Check for Updates… in their app menu. Apps installed with Homebrew: run brew outdated --cask, then brew upgrade --cask <name>. macOS itself updates in System Settings › General › Software Update.
How do I update App Store apps?
Open the App Store and click Updates in the sidebar, then Update All or update one app. To have them update by themselves, turn on Automatic Updates in App Store › Settings.
How do I know if an app has its own updater?
Apps you downloaded from a website often use an updater called Sparkle: they show Check for Updates… in the app's own menu, next to About. Such an app names its update feed in its Info.plist:
defaults read /Applications/iTerm.app/Contents/Info.plist SUFeedURLOther apps, like Chrome or apps built with Electron, update themselves in the background; their About window shows the version and starts a check.
How do I update apps installed with Homebrew?
brew update
brew outdated --cask
brew upgrade --cask firefoxApps that update themselves are left out of that list. brew outdated --cask --greedy includes them too.
Should I always update right away?
For security fixes, yes. For a big new version of an app you rely on for work, reading what changed or waiting a few days can be worth it: now and then a new version is less stable than the one you have.