GitHub
└─ Checking out PRs

It is possible to checkout a pull request directly from the upstream repository via Git:

git fetch upstream refs/pull/$ID/head:$LOCAL_BRANCH
git checkout $LOCAL_BRANCH

An alternative would be to add a new remote for the PR author's fork:

git remote add $FORK $FORK_URL
git fetch $FORK $REMOTE_BRANCH:$LOCAL_BRANCH
git checkout $LOCAL_BRANCH

GitHub└─ Checking out PRs