Nonlinear metasurfaces that dynamically manipulate the phase of a passing light beam are of interest for a wide range of applications. The controlled operation of such devices requires accurate measurements of the optical transmission phase in both the linear and nonlinear regime, an experimentally challenging task. In this paper we show that this phase information can be extracted directly from simple transmission measurements, using a Hilbert transform approach, removing the need for complicated, interferometric experimental set-ups, and enabling direct measurements of the phase in conditions not suitable for other traditional approaches, such Z-scan measurements.