Validation of models for charge transport in organic transistors is fundamentally important for their technological use. Usually current-voltage measurements are performed to investigate organic transistors. In situ scanning Kelvin probe microscopy measurements provide a powerful complementary technique to distinguish between models based on band and hopping transports. We perform combined current-voltage and Kelvin probe microscopy measurements on unipolar and ambipolar organic field-effect transistors. We demonstrate that by this combination we can stringently test these two different transport models and come up with a unified description of charge transport in disordered organic semiconductors.