Which Network is a configuration bridged to?

I have a fenced configuration (virtual network bridged to physical).  How can I find out which physical network it is bridged to, via the API?

I can't see any direct way of getting the name of the network (I can list physical networks with ConfigurationGetNetworks, but which one is used for the bridge?)  I can get the externalIP address, for a machine, so if I could get the network ranges for the physical networks, then I could also work it out from there.

Thanks.