  The existence of strong trigonal warping around the K point for the low
energy electronic states in multilayer (N$\geq$2) graphene films and graphite
is well established. It is responsible for phenomena such as Lifshitz
transitions and anisotropic ballistic transport. The absolute orientation of
the trigonal warping with respect to the center of the Brillouin zone is
however not agreed upon. Here, we use quasiparticle scattering experiments on a
gated bilayer graphene/hexagonal boron nitride heterostructure to settle this
disagreement. We compare Fourier transforms of scattering interference maps
acquired at various energies away from the charge neutrality point with
tight-binding-based joint density of states simulations. This comparison
enables unambiguous determination of the trigonal warping orientation for
bilayer graphene low energy states. Our experimental technique is promising for
quasi-directly studying fine features of the band structure of gated
two-dimensional materials such as topological transitions, interlayer
hybridization, and moir\'e minibands.
