Estimating signal strength from PCM-encoded signal
In general, it is very hard to estimate the power of a signal after it has gone through an analog front-end and sampled. The front-end filters and mixes the signal, which can amplify or attenuate it. It's also common to have an automatic gain control (AGC) stage, which adjusts the signal's amplitude to match the ADC's input range and maximize dynamic range. In many cases, the digital back-end will also normalize the signal amplitude to a pre-defined range, such as -1 to 1.

If you understand every piece in the system very well, you may be able to estimate the signal's power from its samples. Another alternative is to use a scope or a spectrum analyzer to measure the signal as close as possible to the receiver's input.