Reliable and precise measurements of the relative energy of band edges in 2D semiconductors are needed to determine band gaps and band offsets, as well as to establish the band diagram of devices and heterostructures. However, commonly employed techniques such as optical studies and scanning tunnelling microscopy need to be accompanied by modelling for quantitative results. Over the last decade, ionic gate spectroscopy has emerged as a technique that can quantitatively determine the relative alignment of band edges of 2D semiconductors directly from transport measurements. The technique relies on the extremely large geometrical capacitance of ionic gated devices that, under suitable conditions, enables a change in gate voltage to be directly related to a shift in chemical potential. Here, we present an overview of ionic gate spectroscopy and illustrate its relevance with applications to different 2D semiconductors and their heterostructures.