to establish as genuine.

to establish the authorship or origin of conclusively or unquestionably, chiefly by the techniques of scholarship

to authenticate a painting.

to make authoritative or valid.

verb

accredit , attest , authorize , bear out , certify , confirm , corroborate , endorse , guarantee , justify , prove , substantiate , validate , verify , vouch , warrant , back , evidence , testify , demonstrate , establish , show , document