You can tell what direction they are facing, but you need to have your chat log set to show your character's damage. When you hit them with a melee attack it will tell you what side you are attacking from. If you hit it a few times you can tell what all the sides are, although once you find the front or rear you pretty much have everything figured out.

The log will say something like:

Your attack hits the Infernal Nail for X damage.
If it says this you are attacking from the front.

Your attack hits the Infernal Nail from the left/right for X damage.
You are on the side.

Your attack hits the Infernal Nail from the rear for X damage.
You're behind it.

Suirieko is right about them facing a random direction each time though, so make sure you check what side you are on every time before you start combo'ing.