I've never seen the nationality or race indicated on a CMSR. It basically comes down to comparing other sources that confirm, strongly suggest, or support an educated guess.

The "Indian" units of the Indian Territory contain a significant number of white men who lived in IT or were from neighboring states. For example, Capt Hugh L Tinnen of the 4th Arkansas Infantry - State Troops is later Capt, Co. A (second organization) of Watie's 1st Cherokee Mtd Vols. The men in this Co. A were enrolled at Maysville AR by Capt Tinnen and appear on the 1860 Census for Benton County AR. The Cherokee Nation had a number of intermarried white citizens and whites living in the Nation legally e.g. government agents, contract services and labor (clerks, blacksmiths, carpenters, masons, millwrights). Intermarried white citizens were coming into the Nation in the east from the 1700s so there were many "predominately white" mixed blood citizens. There were also Cherokee citizens living in NW Arkansas for various reasons such. Cherokee students went to college in Fayetteville and Cane Hill. Elias C. Boundinot, nephew of Stand Watie, was in a Fayetteville cavalry company at Wilson's Creek.

In this case of John Benge... I ran across the statement that the first Cherokee killed in the war was John Benge at Wilson's Creek. I coincidentally ran across John Benge in the Sebastian County Cavalry killed at Wilson's Creek. I could not find a John Benge in Arkansas on the 1860 US Census or any Benges in or around Ft Smith in county histories. I did know there were a number of Benges in Sequoyah District adjacent to Sebastian and Crawford Counties. I don't have strong evidence but this John Benge might have been a Cherokee that joined an Arkansas unit... and there may have been more but, unfortunately, there is no roster of the company or regiment to review.

What units are you working on the roster for? If these are Indian units, I can help!