Hi Boo,
There's no way to be absoloutly certain, but the first thing to do would be to check the 'which aircraft' section on this site. You'll need to know what flight number you're gonna be on. Look up that flight and it'll give you details of which aircraft have been on that route recently, along with which entertainment system. Like I said, this doesn't guarantee what you'll get, but you should be able to see a trend.
From personal experience though I seem to get something completely against the odds in terms of which plane ever time I fly :S Aircraft type is a reasonably safe bet, but beyond that it's a complete lottery most of the time.
Virgin seem to have cured my girlfriends fear of flying, so I'm sure you'll have a good flight

Graham