Particle methods, despite their inherent capabilities, have a few drawbacks including spurious pressure fluctuations. Several reasons are supposed to be behind this drawback. One of the important issues contributing to unphysical pressure fluctuations in a particle-based simulation of free-surface flows is related to an insufficient or a too simple criterion for assessment of free-surface particles. In this paper, we propose an auxiliary criterion for a more efficient and accurate assessment of free-surface in a particle-based calculation of incompressible free-surface fluid flows. The proposed criterion is based on the nearly symmetric arrangement of non-free-surface particles and its efficiency is demonstrated.