git branch -r
git fetch --all
You can fetch all branches from all remotes like this:
git fetch --all
git branch -r
# If this doesn't work for you, try
git ls-remote --heads <remote-name>
git remote show <remote-name>
git for-each-ref --format=' %(authorname) %09 %(refname)' --sort=authorname
git ls-remote --heads <remote-name>