Maybe there is an rather "easy" solution, NAT Loopback I was told. I run a service on a different machine and try connect to it using the DDNS-Address, where no split-DNS is used. If I can connect, the server should also be reachable from the outside. If I can't connect I am probably on DS-Lite. In my case, client and server are jabber IM, so it is running anyway and that would help me, if this really "works" as intended.