Well, both really. The toadstools or fruiting bodies from honey fungus appear in September, above ground, but the mycelium which produces them is active below ground and in the wood of a tree, particularly the roots of a tree. Think of the mycelium as roots and the toadstools as the flower or fruit, but its the mycelium that do the damage.

welcome to GoY.
If you think you may have it in your garden peel a bit of bark off of a plant that you think is infected and if there is a white 'film' that smells of mushrooms then that tree is infected.

what makes you ask just out of curiosity?