The first way to determine axial symmetry can be done by making use of the definition of the symmetrie parameter. By taking the difference between Vxx and Vyy, we find that the symmetrie parameter has to be zero. Which implies axial symmetrie.
It is also possible to find this by looking at the figure. Here the electronic parts (e-) can be found equidistant from each other, and both lying along the z-axis. Implying axial symmetrie for the electric field gradient tensor.