hey thanks for your question, in this case it's rather arbitrary but conducted by the focal point in the very centre. Since I wanted a symetrical perspective the central vanishing point was obvious and the scene allowed me to do so. Which is not always the case. When you have line, squares and all the perspective is always there and you can't do anything else but follow it.
In the case of your picture here, you have the horizon line and the rock border in front which can give you hints as to a perspective grid. In this case it has to be a bit arbitrary and guessed logically so that it works. See if you can find stuff on perspective on the net 1, 2 or 3 vanishing points, when you understood them, your feeling on the scene will do the rest.
Godd luck dude!!