We can know for sure if we study the principles of linguistics, the science behind languages and its transmission. These principles then get applied to each language and its own forms of logistics to aid a translator and/or interpreter to offer renditions for target audiences.
To translate into ASL you must already KNOW for certain the MEANING of that which you sign.
To translate scripture into another language you must already know for certain the MEANING as well.
None of us can know. That is my point.