Run:
dmsetup ls
This would show something similar to the following:
It says dm-5 is /dev/vg0/tp0 or dm-9 is /dev/vg0/qcow2. The number after : would extend the dm number in /dev.# dmsetup ls
vg0-tp0-tpool (253:4)
vg0-tp0_tdata (253:3)
vg0-tp0_tmeta (253:2)
vg0-qcow2 (253:9)
vg0-tp0 (253:5)