Real-time probing of two-photon absorption is formulated. A two-photon polarizability is introduced and its linear response is derived. The impulse response is shown to depend on the optical phase of incident radiation, and the nonstationary polarizability can be monitored by using a phase-related pair of pulses. Coherent molecular vibration in "dark" electronic states (accessible only through two-photon absorption) can be observed. The results are compared to time-domain observations of coherent vibrations initiated through one-photon absorption or impulsive stimulated Raman scattering. An estimate of the absolute signal intensity is presented.