The unique density of states, and exceptionally low electrical noise, allow graphene-based field effect devices to be utilized as extremely sensitive potentiometers for probing charge transfer with the adsorbed species. In this study we examine charge transfer between graphene and two molecular semiconductors, parahexaphenyl and buckminsterfullerene C60. Through in situ measurements of the current, we directly probe the charge transfer as the interfacial dipole is formed. It is shown that adsorbed molecules do not affect electron scattering rates in graphene, indicating that charge transfer is the main mechanism governing the molecular level alignment at the interface with graphene. From the amount of transferred charge, and the molecular coverage of the grown films, the charge transfer occurring per adsorbed molecule is estimated. In both cases p-type doping with only ~10-3 electrons transferred from graphene per molecule will be show, indicating very weak interaction.