The RasDialer component has a method called GetActiveConnections that will give you any connections that are active on the system. If you want to know what type of entry they are, you can probably look at the type of device used by the connection.
foreach (RasConnection connection in RasDialer.GetActiveConnections())
if (connection.Device.DeviceType == RasDeviceType.Modem)
// The device is a dial-up connection.
Granted there are other devices that support dial-up style systems, you just need to figure out which of those you want to look for. You can also use the RasPhoneBook and look at the entry directly. This method will require a lot more overhead than the previous