A method was proposed for detecting synchronization of two unidirectionally coupled, self-sustained noisy oscillators from a signal of the driven oscillator alone. The proposed method was applied to detect phase locking in vortex flow meters that are applied to noninvasively measure flow velocity of fluids. An advantage of the proposed method is that they cover all frequency ratios. Soft-phase locking is detected by the presence of phase slips, hard phase locking by a suppression of phase difference, while no phase locking is indicated by divergent phase diffusion.