Illustration by Steven Schalla, FlyFishingtheSierra.com, used with permission
As with all cutthroats, check first for the distinct orange-red cutthroat slash below the jaw. The Rio Grande Cutt will exhibit a yellowish gray-green to gray body with and an orange tint along the belly. Spots are medium to large, and become irregular as they approach the tail.
Distinguishing the Rio Grande from its two closest relatives - the Colorado River and Greenback - is difficult as there is substantial overlap in appearance. Colorado River cutthroat trout possess similar coloration, though generally brighter, and with more uniformly large spots. Greenbacks trout tend to have larger spots and more scales in and above the lateral line. The best indication will be location: with few exceptions, any wild cutthroat you catch in the Rio Grande drainage will be an RGCT. However, if you're fishing on a stocked stream, location may not be reliable. Note that there are substantial differences in appearance in RGCT's from drainage to drainage.
Distinguishing Rio Grande from its more northern and western cousins is somewhat easier: a Yellowstone will have a drabber, yellowish brown, silvery, or brassy color body becoming paler by the belly, with medium sized spots; a Snake River Cutt will have drabber body color and many more, smaller spots; a Westslope Cutt will generally exhibit bright but different colors, with a yellow, orange, and red body, and more, smaller spots that won’t appear below the lateral line except near the tail; the Bonneville will have drabber coloration.