btrfs subvolume show /directory/in/question
Most likely you need sudo. The command will succeed if it's a subvolume; it will fail otherwise. You can redirect output to /dev/null and rely solely on the exit status.
This will test every directory available in the directory tree:
sudo find / -type d -exec sh -c '
for d do
btrfs subvolume show "$d" >/dev/null 2>&1 && printf "%s\n" "$d"
done' find-sh {} +
You may want to exclude paths that are beyond suspicion. The following code excludes /proc, /sys and /dev:
sudo find / -type d \( \
\( -path /proc -prune \) -o \
\( -path /sys -prune \) -o \
\( -path /dev -prune \) -o \
\( -exec sh -c '
for d do
btrfs subvolume show "$d" >/dev/null 2>&1 && printf "%s\n" "$d"
done
' find-sh {} + \) \)
Related: How to determine which subvolume a directory/file is on?