No one-word translation. There are several signs (usually compounds) to convey the concept "attest" in contexts.
Meaning: to prove or declare to be true.
E.g. "The fact that you aced the standardized exam attests to the strength of your reading skills.
Meaning: to show something or to say or prove that something is true. E.g. Thousands of Deaf people and hearing allies came out onto the streets to attest their support for the official recognition of ASL in their country.
Meaning: declare that something exists or is the case.
Meaning (law): to sign a document to show that you were present when it was signed by someone else. E.g. The will must be attested by two witnesses.
Meaning: to say officially that something is true or accurate.
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ASL has its own grammar and structure in sentences that works differently from English. For plurals, verb inflections, word order, etc., learn grammar in the "ASL Learn" section. For search in the dictionary, use the present-time verbs and base words. If you look for "said", look up the word "say". Likewise, if you look for an adjective word, try the noun or vice versa. E.g. The ASL signs for French and France are the same. If you look for a plural word, use a singular word.