You can wrap a HTTPClient with the LoggingHttpClient to log all requests generated. This can be accomplished by:
HTTPClient client = HTTPFactoryFinder.createClient(new Hints(Hints.HTTP_LOGGING, "True"));
Or as a system propety:
java -Dorg.geotools.http.logging=True ....
All messages goes through the regular channel, and you will get the url requested. And if you turn on FINEST you can get the content sent in a POST or the content of the response.