This paper discusses the observability of cell stiffness by utilizing a micro-channel whose diameter is slightly smaller than that of a cell. By modeling each cell with distributed springs and dampers, we discuss how many sensing points are necessary and sufficient for evaluating cell stiffness. We define the observability by the velocity of each cell through a micro-channel based upon the experimental observation that the equilibrium velocity of a cell varies corresponding to its stiffness. We conclude that for evaluating cell stiffness, two points are necessary and sufficient while using an infinitely long channel, and three are necessary and sufficient for judging whether cell stiffness is observable or not. This study attempts to provide a practical guidance for developing a simpler, faster, low-cost sensor in the future.