Is there a way to find the NBT of a player head?
Solution 1:
You can throw the head on the ground and use /data get entity @e[type=minecraft:item,sort=nearest,limit=1] to obtain most data contained in the item, including NBT.
Solution 2:
You can also select the item in your hotbar and use /data get entity @s SelectedItem, which reveals only the NBT of the item with less extra data.