Synonyms for Testify:

n.

debunk, deflate, disprove, dispute, repudiate, refute, discredit, disabuse, confound, explode. appeal, appear, call, cite, bring before, stand up for, bring, arraign, allow, swear. depone, law. bear out, justify, back, true. bespeak, mark, argue, point to. testify (noun)
attest, bear witness, prove, evidence, show, take the stand.

v.

affirm (verb)
admit, profess, attest, pronounce, state, assert, express, submit, contend, propound, approve, avouch, assure, verify, adjure, maintain, proclaim, promise, affirm, ratify, pledge, accept, endorse, claim, set down, support, declare, certify, sustain, validate, warrant, aver, acknowledge, corroborate.
disclose (verb)
unmask, reveal, advertise, confess, divulge, brief, disseminate, announce, broadcast, disclose, uncloak, communicate.
evince (verb)
depose, adduce, manifest, document, illustrate, witness, prove, connote, evince, vouch, establish, exhibit, demonstrate, substantiate, observe, confirm, determine, indicate, betoken, exemplify, authenticate.
vouch for; give testimony (verb)
betoken, swear, announce, stand up for, warrant, bear witness, declare, bespeak, attest, certify, indicate, state, argue, affirm, depose, witness, prove, evince, assert, depone, demonstrate, show, point to, corroborate.

Other synonyms:

bespeak, depone. justify. argue. back. bear witness
evidence.
guarantee
stand up for.
indicate
point to.
Other relevant words:
take the stand, argue, evidence, show, swear, bear witness, bear out, mark, depone, back, cite, justify, bespeak, stand up for, point to.

Usage examples for testify

  1. " I'm neither ghost nor thief, as Major O'Shaughnessy will testify when he arrives. – Pixie O'Shaughnessy by Mrs. George de Horne Vaizey
  2. " My shoulder and arm can testify to your zeal," chuckled Lincoln. – The Lost Despatch by Natalie Sumner Lincoln
  3. All these things cost money, as any wholesale merchant engaged in a business where there is sharp competition can testify – Monopolies and the People by Charles Whiting Baker