We have used cross-sectional scanning Kelvin probe force microscopy in air to characterize GaAs/Al_xGa_1-xAs/In_yGa_1-yAs epitaxial layer structures grown for use in heterojunction bipolar transistors. Samples were cleaved in air and microscopy performed on the exposed cross-sections of the epitaxial layers. Surface potential images obtained from these samples reveal contrast that is semi-quantitatively consistent with the expected work function differences between the layers given the known Fermi-level pinning energies on the (110) GaAs surface. These images allow the emitter, base, collector, subcollector and substrate regions to be clearly identified. For measurement conditions under which a large electric field is expected to be present between the tip and sample, variations in contrast are enhanced, allowing device regions and dimensions to be more clearly delineated and suggesting the presence of tip-induced band bending in the sample.