Roadrunner,
There is no one answer, except that the better you can see, the better you can shoot. The amount of light and the angle of the light will make a tremendous difference. Elk on an east slope at sunrise and the west slope at sunset are very easy to distinguish. Elk on the west slope at sunrise and the east slope at sunset can be impossible to see clearly, no matter what the magnification.
At 1000 yds. you will need greater than a 9X to distinguish a mature trophy bull from an average mature bull. A good variable power spotting scope is a must for judging trophies. For a rifle scope, 16X will be good enough and 24X better, but there will be many times that conditions of mirage and glare will make using the higher powers counterproductive. Not every scopemaker's 24X will look the same or have the same apparent resolution. Lens clarity will make a big difference for these long distance looks with less than perfect lighting. Try different scopes from different manufacturers to see which ones you like.
A complication here is that I have found that sometimes I have in the past had to sacrifice image clarity for scope reliability. Scope makers have made tremendous improvements in the last 15 years in both areas. I cannot give advise on brand name, but it makes no sense to have a scope with which you can see a dime at 1000 yds. but it fogs up with the first heavy dew.
[ 08-09-2001: Message edited by: Warren Jensen ]
