We describe a simple method to detect the formation of graphene during Si sublimation from SiC surfaces at elevated temperature. The method exploits differences in the thermionic emission current density between graphene and SiC. When graphene forms, the thermionic current from the sample increases by a factor of about 20. The increase in thermionic emission can be detected in situ using a biased plate located near the sample. The ability to detect when graphene forms during processing is useful in optimizing graphene synthesis processes. © 2011 American Institute of Physics.