  Moir\'e superlattices of two-dimensional (2D) materials with a small twist
angle are thought to exhibit appreciable flexoelectric effect, though
unambiguous confirmation of their flexoelectricity is challenging due to
artifacts associated with commonly used piezoresponse force microscopy (PFM).
For example, unexpectedly small phase contrast ($\sim$$8^{\circ}$) between
opposite flexoelectric polarizations was reported in twisted bilayer graphene
(tBG), though theoretically predicted value is $180^{\circ}$. Here we developed
a methodology to extract intrinsic moir\'e flexoelectricity using twisted
double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For
small twist angle samples, we found that a vectorial decomposition is essential
to recover the small intrinsic flexoelectric response at domain walls from a
large background signal. The obtained three-fold symmetry of commensurate
domains with significant flexoelectric response at domain walls is fully
consistent with our theoretical calculations. Incommensurate domains in tDBG
with relatively large twist angles can also be observed by this technique. Our
work provides a general strategy for unraveling intrinsic flexoelectricity in
van der Waals moir\'e superlattices while providing insights into engineered
symmetry breaking in centrosymmetric materials.
