Haven't tested this, but I suggest looking at java.net.NetworkInterface.getNetworkInterfaces(). This returns an Enumeration of all network interfaces on the machine, or null if there are none.
I'm not sure if it's safe to assume that a non-null response ensures a valid network connection -- depending on your needs, you may or may not need to filter out loopback addresses (which I think you could do with java.net.NetworkInterface.getInetAddresses() on each returned NetworkInterface, and then calling InetAddress.isLoopbackAddress() on each one.)
