What do you look at on the mineral to tell whether an igneous rock formed above or below the surface?

The size of its crystals.

Explanation:

Igneous rocks origin from lava or magma, and are classified into plutonic rocks and volcanic rocks. The first origin after magma has cooled down and solidified under the surface. On the other hand, volcanic rocks consist of lava circulating on the surface that cools down and takes solid form afterward. To identify whether a rock is plutonic or volcanic, we should take a look at the size of the crystals that result from magma and lava cooling down. Plutonic rocks contain large crystals, and volcanic rocks contain small crystals.