Just as easy as Aron's method, but doesn't require you to waste a film. Same thing, just leave the film out, open the back door, set to Bulb and open the shutter (cable release with locks are good to have). Grab a focussing screen from an SLR (if you don't have one lying around you can get one for a Canon or Nikon FF (d)SLR from fleabay for $10. Gently hold the focussing screen over the hole where the film would normally sit, and you can see the entire image as would be captured on the film, with the added advantage of swapping back with the viewfinder in real time. (just make sure your shutter doesn't close while you're working, if it closes you might break it.)