High-frequency modulations appearing in the kinetic energy distribution of laser-accelerated ions are proposed for retrieving the acceleration field dynamics at the femtosecond timescale. Such an approach becomes possible if the laser-cycling field modulates the particle density in the ion spectra and produces quasitime stamps for analysis. We investigate target and laser parameters determining this effect and discuss the dependencies of the observed modulation. Our findings refine a basic mechanism, the target normal sheath acceleration, where an intense and ultrafast laser pulse produces a very strong electrical field at a plasma-vacuum interface. The field decays rapidly due to energy dissipation and forms a characteristic spectrum of fast ions streaming away from the interface. We show that the derived decay function of the field is in accordance with model predictions of the accelerating field structure. Our findings are supported by two-dimensional particle-in-cell simulations. The knowledge of the femtosecond field dynamics helps to rerate optimization strategies for laser ion acceleration.