Simple answer is from cmd "lvs" - the "t" as leading attribute of listed lvm volumes signaled "thin" volume (example shows vol data is one):
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
data pve twi-a-tz-- <53.59g 0.00 1.59
root pve -wi-ao---- <39.41g
swap pve -wi-a----- 8.00g