When it comes to providing evidence of a certain statement or fact, there are various synonyms that can be used in place of "give proof by a witness." Some of the commonly used ones include "attest," "testify," "verify," "validate," "corroborate," "support," "substantiate," "confirm," "establish," "endorse," and "authenticate." Each of these words signifies the act of providing proof or evidence to support a certain claim or allegation, and they can be used interchangeably depending on the context in which they are being used. Whether in a legal or informal setting, using these synonyms can help convey a more nuanced and descriptive meaning to your statements.