Hi,
For example, all connections to standard VPN ports. OpenVPN for instance wraps its packets into some kind of container (that's why you can run OpenVPN and web server on the same port - this is called port sharing).
So it is possible to detect OpenVPN traffic with packet analysis. Some network appliances can already do that. BTW, I think there should be the next step in development of privacy enhaching technologies. First step was development of encryption techniques for the masses (credits to cyphepunks movement in 1990's). Second step was development of anonimisation (Tor). Third step should be development of techniques for hiding encrypted traffic. Obfuproxy, which is a part of Tor package, does that, but we need general purpose standalone tools. Regards, Matej