The search of ultrathin and robust ferroelectrics leads to few promising two-dimensional (2D) materials. Among those, α-phase In2Se3 has drawn a special attention owing to the existence of intercoupled in-plane (IP) and out-of-plane (OOP) ferroelectricity in monolayer form, which makes it a potential candidate for emerging artificial intelligence, information processing and memory applications[1]. Mostly, Piezoelectric force microscopy (PFM) is used to identify the ferroelectricity in 2D materials, where the polarization direction changes under the applied electric field is reflected as phase contrast[2]. However, the contact-mode PFM causes layer damages, in particular if any steps or roughness are present in the topography. Therefore, an alternative non-contact mode characterization technique, like Kelvin-probe force microscopy (KPFM), can be useful to identify the electric field direction in non-destructive way[1]. To provide a proof-of-concept demonstration in device level, we performed KPFM measurements on a ferroelectric-semiconductor field-effect transistors (FeS-FET) under operational conditions[3-5]. Here, we showed a repeatable and reversible IP ferroelectric switching across the device channel, realized by a vertical electric field applied through gate electrode, which also established the inherent orthogonal dipole coupling[3]. Moreover, the interfacial band formations between all the dissimilar materials (including metal electrodes), respective CB and VB offsets, junctions type, effect of ferroelectric dipoles, are deduced via KPFM for coplanar In2Se3-In2O3 heterostructure FETs[4-5]. In conclusion, KPFM is an effective non-destructive method with high spatial resolution over domain orientation, for fast detection of 2D ferroelectricity, which becomes beneficial for research and device innovation.