detecting socket in use on localhost

[Berends]

I am looking for some code to detect whether or not there is already a 
listener to a socket (tcp or udp) on the localhost.

I am writing some software that can be run multiple times on a localhost 
and each will instantiate a listener in a range of ports. I have checked 
several resources (Cookbook, Python Network Programming (Goerzen), 
Programming Python(Lutz)) but cannot find the crux.

Anyone care to point me to some nice methods?

ciao Jochem