Analog backscatter communication is a promising technology for low-power and low-cost communication. The simplicity of the analog backscatter tags make it possible to achieve low-power operation. However, they are limited to perform only simple operations and do not have the capability to perform complex tasks such as estimating the channel parameters. In this work, we propose a novel method to measure the received signal strength at the analog backscatter tag. We achieve this by converting the incident signal strength at the tag, to a frequency to eliminate signal amplitude modifications in the path from the tag to the backscatter receiver. We further enhance the granularity of the signal strength measurement using harmonics that are inherently generated by the backscatter signal. Through experiments, we show that frequency modulation combined with the harmonic spread is a good and a robust indicator of the gain of the carrier-to-tag channel. Our experimental results show a mean error of 1.8% in estimating the received signal power at the tag using backscatter harmonic frequency data.