Fluctuant on palpation indicates the mass contains fluid, which moves under pressure, characteristic of a cystic lesion. In contrast, a hard mass suggests a solid tumor, and fixation implies invasion into surrounding tissues. Overlying skin changes may indicate inflammation or malignancy but do not define cystic nature. Therefore, fluctuance is the key clinical sign that helps differentiate cystic from solid abdominal masses by demonstrating fluid content.

Reference: Bailey & Love's Short Practice of Surgery, Norman S. Williams, et al., 27th Edition.