Attest to is a phrase that means to confirm or validate something. It can be done in a variety of ways, such as certifying, swearing, or avowing. To CERTIFY something is to declare it to be true or genuine, often by signing a document. To SWEAR is to make a solemn promise or vow, usually under oath. To AVOW is to declare openly and firmly, often with pride. All three of these words are synonyms for the phrase 'attest to' and can be used to confirm or validate something.