I'm not immediately familiar with a flag to check for plugins that were compiled in, but you can get a general idea by checking slurm's libraries directory: /usr/lib64/slurm/
So, if you were trying to see if your binary had support for munge, you would check for `auth_munge.so`. If you wanted to see if it had cgroups (and what version), you could look for cgroup
This isn't perfect, but it can at least give you sense for what's available. If you need something specific, it might be best to compile slurm yourself.