Your partner can upload a new version, leave a sketch, or write a comment to let you know about a change. If you have notifications setup, then you'll get an email when any of these happen.
You can also compare changes between versions, which is very useful. For a visual comparison of what is different from one version to the next, see the steps, below.
Go to the 'Files' view and select your file.
Choose 'Compare' from the 3D Viewer menu.
Select which versions you want to compare
Slide the toggle to see changes disappear or appear.
Note: If there is only one version, this won't be available.