If you know or can estimate the distances, use this equation:
Focal Length = Sensor Dimension * Distance / Scene Dimension
Where you match the dimension of the sensor and the scene. e.g:
Focal Length = Sensor Width * Distance / Scene Width
Note that the the advertised size of your sensor is typically not the width, height, or even the diagonal, which are the appropriate dimensions you can use. See this wiki article for the size.
For non-Canon APS-C, the width is 23.6mm. Say your scene / subject is 20m away and you want a width of 5m at that distance. You can use feet as it is just a ratio. This equation says to use a focal length of 93.6mm.
If you are out in the field already and want to know what lens to put on, it might be good to give yourself a good hand calibration:
Hold your fist in front of your face
with your elbow bent at a right angle;
use your knuckles to select the lens
you want for the scene behind your
4knuckles:50mm, 2:100, 1:200.
The trick is figuring out where to
hold your fist so the above rule
holds. Try it a few times.
Take a photo of a scene with a 50mm
lens (or look through the camera's
viewfinder with a 50mm lens in place.)
Put the camera down & hold your fist
in front of your face such that the 4
knuckles just fill that scene height.
Remember where to hold your fist next
time you want to select a lens.