The header may not be utilised, but there is one way to find out: open the side panel and take a peak.
What is odd is the lack of DM entry; I keep the 1394 header disconnected on one of my PCs and the host controller still shows - as it should - in DM.
Is it possible that you need to install it from the driver CD? Or even - given the use of generic cases - that your model does not actually have 1394.
I assume that it doesn't appear in Network Connections?