  We determine the effect of intergalactic magnetic fields on the distribution
of high energy gamma rays by performing three-dimensional Monte Carlo
simulations of the development of gamma-ray-induced electromagnetic cascades in
the magnetized intergalactic medium. We employ the so-called 'Large Sphere
Observer' method to efficiently simulate blazar gamma ray halos. We study
magnetic fields with a Batchelor spectrum and with maximal left- and
right-handed helicities. We also consider the case of sources whose jets are
tilted with respect to the line of sight. We verify the formation of extended
gamma ray halos around the source direction, and observe spiral-like patterns
if the magnetic field is helical. We apply the $Q$-statistics to the simulated
halos to extract their spiral nature and also propose an alternative method,
the $S$-statistics. Both methods provide a quantative way to infer the helicity
of the intervening magnetic fields from the morphology of individual blazar
halos for magnetic field strengths $B \gtrsim 10^{-15}\,{\rm G}$ and magnetic
coherence lengths $L_{\rm c} \gtrsim 100\,{\rm Mpc}$. We show that the
$S$-statistics has a better performance than the $Q$-statistics when assessing
magnetic helicity from the simulated halos.
