The only real "proof" would be either a certificate or a public record of a filing, would it not? I don't know if those exist. My notes (which I think came from the John & Nancy book, but I'm not sure) mention that Nathaniel inherited the family farm from John and that the will had instructions to Nathaniel to care for his mother. I would think that a record of the will being probated, with Nancy as his wife, that would be sufficient "proof" that they were married. However, I don't know if such a record exists.
A portion of my notes: NATHANIEL (NATHAN) CHITTUM Nathaniel (Nathan) Chittum, the youngest child of John and Nancy Sly Chittum, was bornin Goochland County, Virginia on 11 July 1798.He moved with his parents in 1817 to a farm onSouth Buffalo Creek, Rockbridge County, Virginia. On 15 June 1826, Nathan married Elizabeth "Betsy" Deacon (1809-1845), daughter of William and Nancy Peel Deacon of Rockbridge County. The family farm had been bequested to Nathan in his father's will (t834) for him to care for his mother. His mother relinquished her dower for $50 so he could sell the 109 acre family farm.
The (t834) may be a reference to the filing of the will. Maybe someone familiar with the Rockbridge County history can shed some light.