To compare the remote branch you just need to update the remote branch using
git fetch
Then you can differentiate using
git diff <masterbranch_path> <remotebranch_path>
You can use git branch -a to list all branches then choose the branch name from the list from the remote branch name.
Example:
git diff master origin/master (where "master" is a local master branch and "origin/master" is a remote namely origin and master branch.)
Reference: https://git-scm.com/docs/git-diff