A group from the University of Oulu (Finland) has developed and released a set of open source tools for auditing RFID signals. They describe,
We have provided in this toolkit a systematic analysis for the RF protocol of RFID systems. The tools are either written for the Gnuradio software and the USRP software radio hardware or for post processing the extracted data. USRP provides the flexibility of supporting any modulation and frequency (the basic module supports frequencies 0-30 MHz).
You can find project details and download links on their University of Oulu webpage.