This script shows a more complete list of outdated casks. Here's an example:
[nikolay:~] $ brew cask outdated
[nikolay:~] $ ./cask-outdated.sh
caprine (2.6.1) < 2.6.2
dropbox (22.3.22) < latest
github-desktop (0.7.2-cb858085) < 0.8.0-364fa5b9
iterm2-beta (3.1.beta.6) < 3.1.beta.8
microsoft-office (15.33.17040900) < 15.37.17081500
quitter (1.0) < 1.0(107)
sourcetree (2.6.1b) < 2.6.2c