Within Teamwork Desk, you can enable an option to show the last modified information on your help docs.
To enable this, go to the Help Docs area of your site.
Hover over the Help Docs site you wish to update and select Settings from the dropdown menu.
Under the Basics subsection, toggle on the option for Show Date Last Modified.
Once enabled, each article will display the Last Modified details to the top left beneath the help doc name.
For more information, see: Managing Help Docs Settings