On the command line, the Collaborator can use git fetch origin master to get the remote changes into the local repository, but without merging them. Then by running git diff master origin/master the Collaborator will see the changes output in the terminal.
On GitHub, the Collaborator can go to their own fork of the repository and look right above the light blue latest commit bar for a gray bar saying “This branch is 1 commit behind Our-Repository:master.” On the far right of that gray bar is a Compare icon and link. On the Compare page the Collaborator should change the base fork to their own repository, then click the link in the paragraph above to “compare across forks”, and finally change the head fork to the main repository. This will show all the commits that are different.