The analysis of chemical species is one of the most fundamental and long-standing challenges in fiber-optic sensors research. Existing sensor architectures require a spatial overlap between light and the substance being tested and rely either on structural modifications of standard fibers or on specialty photonic crystal fibers. In this work, we report an optomechanical fiber sensor that addresses liquids outside the cladding of standard, 8/125 μm single-mode fibers with no structural intervention. Measurements are based on forward stimulated Brillouin scattering by radial, guided acoustic modes of the fiber structure. The acoustic modes are stimulated by an optical pump pulse and probed by an optical signal wave, both confined to the core. The acoustic vibrations induce a nonreciprocal phase delay to the signal wave, which is monitored in a Sagnac interferometer loop configuration. The measured resonance frequencies and excitation strengths of individual modes agree with the predictions of a corresponding quantitative analysis. The acoustic reflectivity at the outer cladding boundary and the acoustic impedance of the surrounding medium are extracted from cavity lifetime measurements of multiple modes. The acoustic impedances of deionized water and ethanol are measured with better than 1% accuracy. The measurements successfully distinguish between aqueous solutions with 0, 4%, 8%, and 12% concentrations of dissolved salt. The new fiber-sensing paradigm might be used in the monitoring of industrial processes involving ionic solutions.