The high carrier mobility(1,2) and thermal conductivity(3,4) of graphene make it a candidate material for future high-speed electronic devices(5). Although the thermal behaviour of high-speed devices can limit their performance, the thermal properties of graphene devices remain incompletely understood. Here, we show that spatially resolved thermal radiation from biased graphene transistors can be used to extract the temperature distribution, carrier densities and spatial location of the Dirac point in the graphene channel. The graphene exhibits a temperature maximum with a location that can be controlled by the gate voltage. Stationary hot spots are also observed. Infrared emission represents a convenient and non-invasive characterization tool for graphene devices.