CDOT provides a map in PDF format just for cyclists, with info on the roads:

http://www.dot.state.co.us/BikePed/maps.htm

Click on "Legend" to get the map legend, (don't ask me why they put in in a different file),
then click on "State Map" to get the map.


Also, as of last year, Google Streeview has good coverage of the mountains, and when I am considering a new route, I use it to take a look at the roads to see how wide the lanes are, and if there are bike lanes.