For a list of files to be pushed#

$ git diff --stat [remote/branch]

Example:

$ git diff --stat origin/master

For the code diff of the files to be pushed#

$ git diff [remote/branch]

Example:

$ git diff origin/master

To just show names/status#

$ git diff --name-status [remote/branch]

Example

$ git diff --name-status origin/master

See http://stackoverflow.com/questions/3636914/how-can-you-see-what-you-are-about-to-push-with-git for further discussion.