If [remote-path] and [local-path] are the same, you can do
$ git fetch origin master
$ git diff origin/master -- [local-path]
Note 1: The second command above will compare against the locally stored remote tracking branch. The fetch command is required to update the remote tracking branch to be in sync with the contents of the remote server. Alternatively, you can just do
$ git diff master:<path-or-file-name>
Note 2: master can be replaced in the above examples with any branch name