Although overtype mode is disabled by default in Microsoft Word, it can be enabled later at any time, and using Word's main Options menu you can entirely disable the mode. Additionally, you can configure Word to always display the current mode, overtype or insert, in the Status bar.
In the "File" tab, click "Options" and then the "Advanced" section. Under Editing Options, click "Use overtype mode" to remove its check mark. If a check mark displays next to "Use the Insert key to control overtype mode," uncheck that as well.
Displaying the Mode
If you later choose to use the "Insert" key to control overtype mode rather than disable the mode entirely, you can add a notification to inform you which mode you're in. After enabling "Use the Insert key to control overtype mode," right-click the Status bar at the bottom of the window and check the "Overtype" option.