How can I estimate reception signal strength for RFM12 or RFM69?
Both the RFM12 and RFM69 modules provide an output indicating the current RSSI value (received signal strength).

RFM12: pin 15 for analog output, bits 2-0 for setting the digital RSSI threshold (power setting command), RSSI bit in the status register for checking the signal level.

RFM69: RssiThreshold Setting in RegRssiThresh, and RssiValue for reading the value (provided it is above the set threshold).

Some links with further information:

http://talk.jeelabs.net/topic/385 and http://jeelabs.org/2010/06/27/rfm12b-as-spectrum-analyzer/ for using the RFM12 module as spectrum analyzer

https://hallard.me/rfm12b_arssi-library/ and http://madscientistlabs.blogspot.nl/2014/01/more-than-one-way-to-skin-cat.html for libraries to read the RSSI values