Next time you should just push your changes from your branch to the remote repository, then submit a "pull request" for the branch back into master where the code can be reviewed prior to merging.
When you push changes to your branch, to compare your changes, go to that branch and look for this near the top in the code tab:
This is some good reading as well about how/when to use forking & pull requests: https://help.github.com/articles/using-pull-requests
EDIT:
And since you did say this is after the fact, the other thing you can do is go to the master branch->commits section, and click on the commit where you merged your branch in. That page allows you to make comments and view the changed files, so you can still review your code before you actually push it to your server. However, you should still do the other way next time.