Synonyms for Attest:

n.

disprove, bear out, disabuse, confound, discredit, deflate, explode, back, true, justify, dispute, refute, repudiate, debunk. cause, assignment, mark, bail, bespeak, cancel, cancellation, assign, point to, argue, blind trust, alienation, alienate. depone, swear, law. attest (noun)
take the stand.

v.

affirm (verb)
assert, warrant, aver, declare, proclaim, claim, adjure, sustain, express, validate, propound, contend, accept, testify, affirm, acknowledge, pronounce, profess, certify, endorse, pledge, ratify, promise, set down, submit, state, avouch, maintain, verify, admit, support, approve, assure, corroborate.
affirm, vouch for (verb)
aver, witness, confirm, exhibit, authenticate, prove, ratify, swear, indicate, adjure, support, corroborate, argue, sustain, substantiate, bear out, certify, testify, demonstrate, verify, warrant, assert, declare.
communication (verb)
certify, manifest, demonstrate.
evince (verb)
confirm, connote, evince, exemplify, witness, authenticate, exhibit, determine, prove, substantiate, illustrate, demonstrate, betoken, adduce, observe, indicate, establish, depose, vouch, manifest, document.

Other synonyms:

assignment, blind trust, assign, bespeak, depone, cancel, bail. alienation, swear, cancellation. alienate. cause. justify. argue. back. indicate
point to, bespeak.
manifest
evince.
promise
assure.
Other relevant words:
mark, bail, swear, bear out, bespeak, depone, justify, back, point to, assignment, take the stand, argue, alienate.

Usage examples for attest

  1. In the desolation of her last days, when she bids her attendants go to her sister and " Tell her to come and close my dying eyes And wear my crown and dance upon my grave," Mary, to attest her misery, seats herself on the ground, like Constance in " King John"; and the comment of one of her women hereupon is strikingly picturesque: " Good Lord! – Views and Reviews by Henry James
  2. It was wished to retemper for him the sword of the constable Duguesclin; and it was hoped that a statue erected to his honour would at once attest to posterity his spotless glory and the gratitude of the Bourbons. – The Project Gutenberg Memoirs of Napoleon Bonaparte by Bourrienne, Constant, and Stewarton