I managed a quick, incomplete "circle tour" last fall.
From Montrose to Cimarron, once you get up into the hills toward Cerro Summit you can see the roadbed pretty good. The road follows the roadbed.
Of course between Montrose and Gunnison most the roadbed is under hundreds of feet of water. I can't remember if we could see the roadbed west of Gunnison or not.
East of Gunnison going toward Monarch Pass you can make out roadbed, at least until it heads toward Marshall Pass. On the otherside of Monarch you can make out the Monarch branch pretty well.
I hope to return to Colorado some year and more intensly seek out Marshall and RGS roadbeds.