There are a few files on X10 protocol here:
http://www.davehouston.net/Basically you send an X10 Address command matching the RR501s.
Then House Code Status Request.
It will answer back.
Here is a sample capture from an RR501 and an Insteon 2456S3 switch with a X10 address in it.
You will see requested the status.
Then turned it on.
Requested the status again.
Smarthome 1132CU controller and Smarthome Essentials Software.
RR501 Address A9:
T: A9 - 10:28:24 AM 3/11/2011
T: AStatus Request - 10:28:28 AM 3/11/2011
R: AStatus=Off - 10:28:28 AM 3/11/2011
T: A9 - 10:28:33 AM 3/11/2011
T: AOn - 10:28:34 AM 3/11/2011
T: A9 - 10:28:39 AM 3/11/2011
T: AStatus Request - 10:28:41 AM 3/11/2011
R: AStatus=On - 10:28:42 AM 3/11/2011
Insteon 2456S3 ApplianceLinc X10 address A15:
T: A15 - 10:28:45 AM 3/11/2011
T: AStatus Request - 10:28:52 AM 3/11/2011
R: A15 - 10:28:52 AM 3/11/2011
R: AStatus=Off - 10:28:53 AM 3/11/2011
R: A15 - 10:28:53 AM 3/11/2011
R: AOff - 10:28:54 AM 3/11/2011
T: A15 - 10:28:58 AM 3/11/2011
T: AOn - 10:28:59 AM 3/11/2011
T: A15 - 10:29:04 AM 3/11/2011
T: AStatus Request - 10:29:05 AM 3/11/2011
R: A15 - 10:29:06 AM 3/11/2011
R: AStatus=On - 10:29:06 AM 3/11/2011
R: A15 - 10:29:07 AM 3/11/2011
R: AOn - 10:29:07 AM 3/11/2011