Glass, glass, and glass some more. If you see no sign, check out another drainage. I usually look for good sized pieces of dark timber on the upper third of a slope and think to myself where they might be feeding. Concentrate on the area in between those spots early morning and just glass. If you know of any access points below you where hunters are coming in you can look at the terrain search out corridors or saddles where elk may leave one drainage and head to the next. I've hunted CO many years and honestly have filled my tag much more often than not, the only tags I didn't punch were archery and that was about 50%, rifle bull elk I've filled every one of em (6 so far). If what your doing isn't working I can't stress enough you need to change tactics and go find them. If you know where your camp will be, start on Google Earth and get yourself familiar with the terrain and cover now. You should show up at camp and know what's where and where you want to start. I always take two rifles, one to 'hunt' with that's capable of 600 yards if need be and light enough to carry easily, and the other is a 30" barreled 300 RUM shooting 200gr accubonds @ 3250. That one has taken elk to 850 and a mule deer at 1180. Most elk I've shot have been at 300 or less with the exception of the last several years when I started playing with that 300. If you can effectively shoot to 800 it really does open up a lot of opportunity in the western states.
