Lua Code:
local isSelected = C_AzeriteEmpoweredItem.IsPowerSelected(azeriteEmpoweredItemLocation, powerID)
Quick example to check for Arcane Pummeling (88) on the helmet (1):
Lua Code:
/dump C_AzeriteEmpoweredItem.IsPowerSelected(ItemLocation:CreateFromEquipmentSlot(1), 88)