If it's a hosted family you could test its Host property (this will throw an exception if it isn't hosted). Or maybe you could check its FacingOrientation (though this depends on how the family is made): a vector like (0,0,1) indicates it's facing up, a vector like (0,0,-1) means it's facing down, and a vector in the xy plane means it's facing horizontally.
Though it's possible to create a non hosted family for a light fixture and when you place it on a work plane to represent it hanging from/on the ceiling, its FacingOrientation is actually horizontal because it was created already facing down.
