Open the document.
If there are new changes from other users, the “Show all open changes to the document” tab appears on the left side of the tab bar. The red highlighted number indicates the number of sections containing new changes.
Click on it and you will see an overview of all sections with the new changes.
A small “Click to automatically open section history” button is in the upper right-hand margin. When you click this, the section history is also shown next to the corresponding section as soon as you click on an entry in the overview.
This way you can quickly see at a glance who added the new changes.
You can also open the section history by clicking on the small number to the right of the section. More information about this is found here.