  Recent studies have suggested that the Milky Way (MW)'s Dark Matter (DM) halo
may be significantly tilted with respect to its central stellar disk, a feature
that might be linked to its formation history. In this work, we demonstrate a
method of constraining the orientation of the minor axis of the DM halo using
the angle and frequency variables. This method is complementary to other
traditional techniques, such as orbit fitting. We first test the method using a
simulated tidal stream evolving in a realistic environment inside an MW-mass
host from the FIRE cosmological simulation, showing that the theoretical
description of a stream in the action-angle-frequency formalism still holds for
a realistic dwarf galaxy stream in a cosmological potential. Utilizing the
slopes of the line in angle and frequency space, we show that the correct
rotation frame yields a minimal slope difference, allowing us to put a
constraint on the minor axis location. Finally, we apply this method to the
Sagittarius stream's leading arm. We report that the MW's DM halo is oblate
with the flattening parameter in the potential $q\sim0.7-0.9$ and the minor
axis pointing toward $(\ell,b) = (42^{o},48^{o})$. Our constraint on the minor
axis location is weak and disagrees with the estimates from other works; we
argue that the inconsistency can be attributed in part to the observational
uncertainties and in part to the influence of the Large Magellanic Cloud.
