The phrase "attest to" refers to giving evidence or proof of something or acknowledging that something is true or genuine. Some alternative synonyms for "attest to" include affirm, verify, certify, confirm, substantiate, validate, endorse, vouch for, bear witness to, declare, avow, indicate, testify, and assert. These words can be used to convey a similar meaning and express the same idea as "attest to." Each synonym has a slightly different connotation and should be chosen based on the context in which it is used. Regardless of which synonym you choose, the meaning will convey a strong sense of confidence and certainty in the truth of the matter being discussed.