Show mounted btrfs subvolumes?

findmnt -nt btrfs, the source subvolume is in [...], the mountpoint is the first column. Alternatively, you could look into the file /proc/self/mountinfo yourself.

On older systems that don't have the nicely-formatted findmnt command, just run mount with no arguments. It will spit out a list of devices, mountpoints, types and options. Pipe it to grep for btrfs if you want to filter it down for easy reading.