'git diff' between a remote and local repository

I am trying to diff my local file with a GitHub repository before I submit a pull request, so I can see what will show up. Is there an accurate way of doing this?

I assume GitHub's compare tool manipulates Git's diff.

Don't do a pull :

• do a fetch (the syntax is the same as git pull, but it doesn't automatically merge)
• do a diff between your dest branch and the other branch
• then do a merge if you want