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 $0.1lesssimkappalesssim4.6$, whereas profiles with a smooth-edged core have a narrower range of $0.1lesssimkappalesssim2.2$ 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.
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