The Collaborative International Dictionary of English v.0.48:

Attest \At"test"\, v. t. [imp. & p. p. Attested; p. pr. & vb.
   n. Attesting.] [L. attestari; ad + testari to bear witness:
   cf. F. attester.]
   1. To bear witness to; to certify; to affirm to be true or
      genuine; as, to attest the truth of a writing, a copy of
      record.
      [1913 Webster]

            Facts . . . attested by particular pagan authors.
                                                  --Addison.
      [1913 Webster]

   2. To give proof of; to manifest; as, the ruins of Palmyra
      attest its ancient magnificence.
      [1913 Webster]

   3. To call to witness; to invoke. [Archaic]
      [1913 Webster]

            The sacred streams which Heaven's imperial state
            Attests in oaths, and fears to violate. --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

attested
    adj 1: established as genuine [syn: attested, authenticated,
           documented]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "attested":
   actual, affirmed, alleged, announced, ascertained, asserted,
   asseverated, assured, authenticated, averred, avouched, avowed,
   borne out, categorically true, certain, certified,
   circumstantiated, confirmed, corroborated, decided, declared,
   demonstrated, deposed, determinate, determined, documentary,
   effectual, enunciated, established, factual, fixed, guaranteed,
   historical, in the bag, made sure, manifestoed, nailed down,
   not in error, objectively true, on ice, open-and-shut, pledged,
   predicated, professed, pronounced, proved, proven, real, secure,
   set, settled, shown, stated, substantiated, sure-enough, sworn,
   sworn to, tested, tried, true, true as gospel, truthful,
   unconfuted, undenied, undoubted, unerroneous, unfallacious,
   unfalse, unmistaken, unquestionable, unrefuted, validated,
   veracious, verified, veritable, vouched, vouched for, vowed,
   warranted