The two-frequency wave function of biphotons generated in the process of spontaneous four-wave mixing in a photonic crystal fiber is studied. A method for estimating the degree of entanglement of biphotons by time-frequency variables is presented and tested experimentally. The degree of spectral anticorrelation of a two-photon wave packet, which can be used as an alternative to the Schmidt and Fedorov parameters for estimating spectral entanglement, is revealed. The results on two-photon interference in a Mach–Zehnder interferometer are used to determine the parameter of spectral anticorrelation for the state generated in the four-wave mixing process in the photonic crystal fiber.