The Collaborative International Dictionary of English v.0.48:

Truthful \Truth"ful\, a.
   Full of truth; veracious; reliable. -- Truth"ful*ly, adv.
   -- Truth"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

truthful
    adj 1: expressing or given to expressing the truth; "a true
           statement"; "gave truthful testimony"; "a truthful
           person" [syn: truthful, true] [ant: untruthful]
    2: conforming to truth; "I wouldn't have told you this if it
       weren't so"; "a truthful statement"

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "truthful":
   accurate, actual, ascertained, attested, authenticated, candid,
   categorically true, certain, certified, confirmed, corroborated,
   demonstrated, determined, documentary, earnest, effectual,
   established, factual, faithful, forthright, frank, historical,
   honest, not in error, objectively true, proved, real, realistic,
   reliable, sincere, straightforward, substantiated, sure-enough,
   true, true as gospel, true to life, true-meaning, true-speaking,
   true-tongued, trustworthy, truth-bearing, truth-declaring,
   truth-desiring, truth-filled, truth-guarding, truth-loving,
   truth-passing, truth-speaking, truth-telling, unconfuted, undenied,
   undoubted, unembellished, unerroneous, unfallacious, unfalse,
   unmistaken, unquestionable, unrefuted, unvarnished, validated,
   veracious, veridical, verified, veritable



The Devil's Dictionary (1881-1906):

TRUTHFUL, adj.  Dumb and illiterate.