In this paper, we investigate the primary-side control of a Wireless Power Transfer (WPT) link, i.e., the capability of delivering the optimal power level to the load without compromising system efficiency, by only sensing quantities available at the primary side. When adding a simple power regulator at the secondary side, we observe and analyze the existence of a Maximum Efficiency Transfer point, easily detectable at the primary side, and corresponding to the additional power regulator being barely on, with negligible dissipated power. More important, the approach does not require any advanced system modeling, nor the estimation of the system parameters. A theoretical model is developed and verified by measurements on a prototype built upon a 60 mW-class-E based WPT link working at 6.78 MHz.