  Of concern is the a priori symmetry of traveling wave solutions for a general
class of nonlocal dispersive equations \[ u_t +(u^2 +Lu)_x =0, \] where $L$ is
a Fourier multiplier operator with symbol $m$. Our analysis includes both
homogeneous and inhomogeneous symbols. We characterize a class of symbols $m$
guaranteeing that periodic traveling wave solutions are symmetric under a mild
assumption on the wave profile. Particularly, instead of considering waves with
a unique crest and trough per period or a monotone structure near troughs as
classically imposed in the water wave problem, we formulate a reflection
criterion, which allows to affirm the symmetry of periodic traveling waves. The
reflection criterion weakens the assumption of monotonicity between trough and
crest and enables to treat a priori solutions with multiple crests of different
sizes per period. Moreover, our result not only applies to smooth solutions,
but also to traveling waves with a non-smooth structure such as peaks or cusps
at a crest. The proof relies on a so-called touching lemma, which is related to
a strong maximum principle for elliptic operators, and a weak form of the
celebrated method of moving planes.
