  We investigate the shape of the jet break in within-beam gamma-ray burst
(GRB) optical afterglows for various lateral jet structure profiles. We
consider cases with and without lateral spreading and a range of inclinations
within the jet core half-opening angle, $\theta_c$. We fit model and observed
afterglow lightcurves with a smoothly-broken power-law function with a
free-parameter $\kappa$ that describes the sharpness of the break. We find that
the jet break is sharper ($\kappa$ is greater) when lateral spreading is
included than in the absence of lateral spreading. For profiles with a
sharp-edged core, the sharpness parameter has a broad range of
$0.1\lesssim\kappa\lesssim4.6$, whereas profiles with a smooth-edged core have
a narrower range of $0.1\lesssim\kappa\lesssim2.2$ when models both with and
without lateral spreading are included. For sharp-edged jets, the jet break
sharpness depends strongly on the inclination of the system within $\theta_c$,
whereas for smooth-edged jets, $\kappa$ is more strongly dependent on the size
of $\theta_c$. Using a sample of 20 GRBs we find nine candidate smooth-edged
jet structures and eight candidate sharp-edged jet structures, while the
remaining three are consistent with either. The shape of the jet break, as
measured by the sharpness parameter $\kappa$, can be used as an initial check
for the presence of lateral structure in within-beam GRBs where the afterglow
is well-sampled at and around the jet-break time.
