A new criterion is proposed for a more accurate and efficient assessment of free-surface in particle methods. The new criterion is simply based on the fact that for a non-free-surface particle located inside the computational domain a nearly symmetric distribution of neighboring particles exists. The efficiency of the new criterion has been shown by simulating a hydrostatic pressure calculation by MPS and MPS-ASA (MPS with the Assessment of free-surface based on nearly Symmetric Arrangement of non-free-surface particles) methods (Fig. 1) and a dam break simulation by the CISPH-HS and CISPH-HS-ASA methods (Fig. 2).