Nowadays everything in this SOA age (Service Oriented Arhitecture) is developed as webservices.
There's a lot of frameworks that hides the specific implementation so often you ask:
What do I send to that webservice, and what do the service return?
For that sake, there's a fantastic tool called 
tcpmon, which can be started as 
Java Webstart.
Here you make a proxy where you specify a localport where tcpmon listens, and which server/port the real service is implemented. Tcpmon then shows exactly what is transmitted to the service and what is received.
Right now I have the problem that we're using a proxy, so tcpmon can't see the destination service (it's through VPN).
I can see the server in my browser, so I looked up the proxy in my browsers configuration 
(was webproxy:8080) and tried starting up tcpmon with 
-Dhttp.proxyHost=webproxy -Dhttp.proxyPort=8080, but still it won't work...
The solution is actually very simple:
- Start the application with: 
-Dhttp.proxyHost=localhost -Dhttp.proxyPort=8888 (Now we use tcpmon as proxy) 
- In tcpmon start a new connection with: (the proxy now uses a proxy :-)
 | Port | 8888 | 
  | Server Name | Proxyserver (in my case webproxy) | 
| Server Port | Proxyserverport (in my case 8080) | 
 
You now don't have to change the endpoint of the service, because it automatically routes though tcpmon!
Voila! Mission accomplished!