Perhaps (though doubtful) you could verify if there are no backdoors in the chip design, microcode or firmware source. But that's not sufficient since you can'

Perhaps (though doubtful) you could verify if there are no backdoors in the chip design, microcode or firmware source. But that's not sufficient since you can't reasonably verify if a particular piece of hardware has the design, microcode or firmware that it is supposed to have. For all you know there's a tricky modification there that's not in the documents you got. There is published research on nearly undetectable alterations of chips during manufacturing, altering its functionality; chips can lie about the firmware they have, etc.