Truthful, a. — Webster enters it under “truthful”
Full of truth; veracious; reliable. -- Truth"ful*ly, adv. -- Truth"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for truthfulness; he lists it under veracity.
Where he writes about it:
Truthfulness is a quality that may inhere either in a person or in his statements or beliefs. — from veracity
Candor, frankness, honesty, and ingenuousness are allied with veracity, and verity with truth, while truthfulness may accord with either. — from veracity