For a list of files to be pushed, run:
git diff --stat --cached [remote/branch]
example:
git diff --stat --cached origin/master
For the code diff of the files to be pushed, run:
git diff [remote repo/branch]
To see full file paths of the files that will change, run:
git diff --numstat [remote repo/branch]
If you want to see these diffs in a GUI, you will need to configure git for that. See How do I view ‘git diff’ output with a visual diff program?.