It is shown that in stimulated Brillouin scattering of picosecond laser pulses the proportion of the energy scattered in a strongly inhomogeneous short dense-plasma layer reaches a fraction of a percent at intensities below 2 × 10¹⁵ W cm^-2 in the absence of a prepulse or less than 2 × 10¹⁴ W cm^-2 if a prepulse is present. This can be used to detect a prepulse and to obtain an experimental estimate of the contrast. In the case of longer (∼10ps, ∼2 × 10¹⁵ W cm^-2) pulses the proportion of the energy scattered by the stimulated Brillouin process may reach several percent in the absence of a prepulse.