  Measurements of the Fermi surface are a fundamental technique for determining
the electrical and magnetic properties of solids. In 2D systems, the area and
diameter of the Fermi surface is typically measured using Shubnikov-de Haas
oscillations and commensurability oscillations respectively. However, these
techniques are unable to detect changes in the parity of the Fermi surface
(i.e. when +k $\neq$ -k). Here, we show that transverse magnetic focussing can
be used to detect such changes, because focussing only measures a well defined
section of the Fermi surface and does not average over +k and -k. Furthermore,
our results show that focussing is an order of magnitude more sensitive to
changes in the Fermi surface than other 2D techniques, and could be used to
investigate similar Fermi surface changes in other 2D systems.
