The rule of thumb is that approximately one-quarter of the viewfinder area is about the same as a focal length that's double what you're using. For example, if you've got a 24mm lens on the camera, split the viewfinder horizontally and vertically and one of those four boxes you just made is about what you'd get with a 50mm lens. This isn't a perfect predictor, but it's close enough to get yourself to the approximate lens you need.