This is a probe that can be used to measure any curl on a stack of sheets in a sheet stacking tray. In printing systems including electrophotographic systems, sheets tend to curl on edge portions. This curl can adversely affect the final copy and copies fed to finishing stations.This probe lightly contacts the upper stack surface and detects sheet curl especially on the edge portions of the stack. The probe measures deviations of the surface as compared to a starting point defined on the terminal edge of the stack. The information provided by the probe is then fed to a user or controlled for transmission to a correcting apparatus.