Do you mean surveying control marks or bench marks? I assume that it depends on whether they were ever surveyed to the standard. It's possible the marks have been removed or destroyed. You can see if there should be anything by using the NGS datasheet page. The descriptions will also describe how to find them and when last found.
+1 for surveying control marks or bench marks?
(N.B I got these images off the internet)
The two types have different information written on them and in the UK look different...I know in the UK they are now being left until they are destroyed by erosion or the land being redeveloped. not much help as you're looking at Colorado... 🙂 but I guess they could have been destroyed or could have not been built.