In the Collaborate toolbar, you can get an overview of all the changes that have been made. If you hover over a single change, an eye icon will appear.
To open the file in your diff tool and highlight the changes, click the eye icon.
Ensure you have a diff tool installed and linked up to Unity. If you do not already have one, DiffMerge is free and cross-platform.
To use a diff tool with Unity, go to the Preferences menu, click External Tools, and change the diff tool via the drop-down menu next to Revision Control Diff/Merge.
Note: The eye icon appears after pulling another team members' changes and the file is different from your local copy. Unity Collaborate does not support performing a diff on two files that have already been committed.