When a Data Shortcut or an External Reference has been updated, there should be a notification bubble pop up on your screen at the bottom left. 

This bubbles behaviour is controlled by the command TRAYSETTINGS, so if it doesn't show up (or if you want to change the behaviour of it) start looking at the settings in TRAYSETTINGS. 

But if the bubble still doesn't show up, and you have two or more screens, that might be an issue for you. If you use Civil 3D on the screen that is not set as the Primary Screen - this bubble doesn't show up. You can move Civil 3D over to the Primary Screen - or change which screen is Primary in your Windows Display settings.