We propose here a fully-passive wireless liquid sensor using a harmonic transponder, which comprises a dual-band microstrip antenna reconfigured by different types of liquids injected in a fluidic cavity. Different from traditional radio-frequency (RF) backscatter sensors, the proposed harmonic-transponder sensor (or harmonic sensor) receives frequency-hopped RF monotones and backscatters their second harmonics, with the peak frequency shifted by dielectric properties of liquid mixtures. This microstrip antenna has a hybrid-feed structure, of which an outer split-ring patch exhibits a narrow-band TM310 mode at the fundamental frequency (f0) and an inner elliptical patch displays a wideband resonance centered at the second-harmonic frequency (2f0), achieved with hybridization of TMe110 and TMo110 modes. In particular, the outer split-ring patch is loaded with a fluidic channel system to tune the resonance frequency of the TM310 mode (f0). We demonstrate that the type of liquid mixture filling in the fluidic cavity can be clearly perceived by reading the peak received signal strength indicator (RSSI) in the spectrum of second harmonics. Our results show the potential for deploying this passive wireless sensor in noisy environments that include clutters, multiple reflections, jamming, and crosstalks.