It’s normally easy to tell which direction a model is facing – he’ll (it’ll?) be aligned squarely with one of his base’s edges.
Sometimes, however, it’s not so clear. This doesn’t tend to be a problem in a unit, where all the models are assumed to face the same way (and a command group can normally be found in the front rank). If you have a single model, such as a monster or character, whose facing is not obvious, make sure that your opponent can tell which way it’s facing – it could prove crucial in the battle!