The webdriver read-only property of the navigator interface indicates whether the user agent is controlled by automation.

It defines a standard way for co-operating user agents to inform the document that it is controlled by WebDriver, for example, so that alternate code paths can be triggered during automation.

The navigator.webdriver property is true when in:

Chrome

The --enable-automation or the --headless flag or the --remote-debugging-port is used.

Firefox

The marionette.enabled preference or --marionette flag is passed.

A Boolean