In most cases the below command will work:
git branch -r
But for some git branch -r does not work. Then maybe you've modified the default refspec, so that git fetch and git remote update don't fetch all the remote branches.
If git branch -r does not work try:
git ls-remote --heads <remote-name>
Or you can check with this git command
git config --get remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
contains a wildcard (*), of course, means everything under that path.