I've done exactly that to confirm that the ARF is spitting out data, but not to decode the data itself, but this was for my own use (not as a test for the PilotAware connected ARF, but it should be exactly the same). The same RTL-SDR can be used as a crude spectrum analyser and you can quite quickly check that working using some readily available software.
I'm using Linux and there's package called Gqrx suits that. I've not tried but this:
http://sourceforge.net/projects/guiforrrtlpower/ seems to be a similar thing for Windows.
All I did was plug in the RTL-SDR dongle and tune the Spectrum Analyser to 869.4Mhz in Normal FM mode with a Medium AGC speed. Bingo! - there was the data being splatted out at 2 second intervals. Stops when you pull the power. That was showing that something was being sent.
Are you thinking of using a second RTl_SDR to decode the P3I sentences? why?