Verb (used with object) : to attest the truth of a statement. ,His works attest his industry.    From Dictionary.com. 

Verb (used without object) : to attest to the reliability of an employee.    From Dictionary.com. 

Chancelier a du vous attester, ne m 'avois permis de vous rappeller.    From Wordnik.com.  [Pickle the Spy; Or, the Incognito of Prince Charles] Reference

In that case I did not fear to make him the attester of my innocence.    From Wordnik.com.  [Arthur Mervyn Or, Memoirs of the Year 1793] Reference

His relation to the truth which He brings to us is not that of a man who has thought it out, who has been brought to it by experience, or by feeling, or by a long course of investigation; still less is it the relation which a man would bear to a truth that he had learnt from others originally, however much he had made it his own thereafter: but it is that of one who is not a thinker, or a learner, or a reasoner, but who is simply an attester, a witness.    From Wordnik.com.  [Expositions of Holy Scripture Second Corinthians, Galatians, and Philippians Chapters I to End. Colossians, Thessalonians, and First Timothy.] Reference

Of this truth you're the fittest attester.    From Wordnik.com.  [Memoirs of the Life of the Rt. Hon. Richard Brinsley Sheridan — Volume 02] Reference