Is there any way to tell whether it is the a-leg or b-leg when receiving the POST data from mod_xml_cdr?

I wrote a parser in Python which parses the a_*.cdr.xml log files, and only grabs the info in the associated b-leg log files if it needs additional information(for example - in the case of a call that ends with mod_fifo).  I want to adapt this to using the POST features of mod_xml_cdr so that it can be real time.

Any suggestions are greatly appreciated,

-Adam