For starters, the firewire port has to work, by law. Search on these forums and you'll find some concrete references to it. I don't own a D-VHS but one way you can fiddle around is to simple attach a firewire cable from your STB to your computers (XP and above) and try to get it running using these drivers--instruction and drivers on this page http://replayguide.sourceforge.net/dct6412/
This could at least help you tell for sure if it's on or not.
I've read posts where some people found two firewire ports on the back of their unit, one live and one dead, so be sure to check both.
Also, be sure to set the STB to a 'network' channel (ABC, NBC, CBS) etc, to avoid any 5C problems when using a computer--this is a problem you won't have a D-VHS unit.
A side note: if you have a Mac running OSX, this all much easier and foolproof. If you have one, or access to one, just install the firewire SDK from the Apple site, and use AVCVideocap (included with the SDK) to test.