Among various configurations for fiber-optic distributed strain and temperature sensors, Brillouin optical correlation-domain reflectometry (BOCDR) is known to have high spatial resolution, random accessibility to measurement points, and single-end accessibility. The spatial resolution in BOCDR relies on accurately determining the modulation amplitude of the light, which is inversely proportional to the product of the modulation amplitude and frequency. Existing approaches for modulation amplitude measurement in BOCDR involved observing the modulated light spectrum using an optical spectrum analyzer (OSA) or employing a separate heterodyne detection system with an electrical spectrum analyzer (ESA). However, OSA-based methods suffer from limitations in frequency resolution, high costs, and bulky size, while using a separate heterodyne detection system requires modifications to the BOCDR setup, reducing convenience. In this study, we present a novel method for measuring modulation amplitude in BOCDR without modifying the experimental setup. Our approach utilizes the spectral width of noise caused by Rayleigh scattering, enabling modulation amplitude measurement without length constraints on the sensing fiber. By directly observing the Rayleigh-noise components using the existing BOCDR setup and an ESA, our method provides high-frequency resolution and straightforward implementation. This significantly enhances the convenience of modulation amplitude measurement, facilitating accurate spatial resolution evaluation in BOCDR.