I used to think the answer was to use a Hollywood director's finder in one of its wide-screen modes, but these days there's a more interesting way: use your iPhone. I use a Pano app on the iPhone to quickly generate a handheld pano that's wider than I think I'll eventually want, then review it to find where I want my final start/end points to be (bonus credit: edit the pano you made in one of the iPhone photo editing apps to see the exact pano you want to create on the DSLR).
If you know something about the optics on the iPhone and your DSLR, you'll even be able to guess at how many stitches it'll take to do your final pano stitch sequence on the DSLR.
Another alternative is to use one of the compact cameras with a "sweep pano" function (such the Sony RX-100). Why not just use the iPhone (or compact) and ditch the DSLR for panos? It all boils down to quality and pixels. On my D3x I've generated massive panos (a two-row, five-column stitch can easily be 15,000 x 9,000 pixels in size, or 135mps), and they're very high in quality.