Add fetch = +refs/pull/*/head:refs/remotes/origin/pr/* in your .git/config file:

[remote "origin"]
    url = https://github.com/[user_or_org]/[repository].git
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/pull/*/head:refs/remotes/origin/pr/*

In this example, origin is the main repository, not the forked one.

And then git fetch origin and git checkout pr/[ID] to review changes.

source: https://gist.github.com/piscisaureus/3342247