How, do you know if you or someone else really knows a foreign language or an L2? Easy, if you or another person can speak, listen, and (read) write the language in such a way that you can communicate back and forth with native speakers and very proficient experienced L2 or L3 users of the said language then you know that you (or someone else) really has acquired a high level of fluency in the said language.
To be sure while Classical or Biblical Hebrew is not identical with modern Hebrew never the less there is a very great overlap between them. So, much so that young native speakers of modern Hebrew can easily read the narrative sections of the Hebrew Bible and more proficient educated speakers of modern Hebrew can read all of the Hebrew Bible just the same as a modern speaker of English can read the works of William Shakespeare. Is the ‘middle English’ that Shakespeare uses the same as the current English of today? No, absolutely not both in terms of grammar and some vocabulary items, however, a literate native speaker of English can generally read and understand the works of Shakespeare.
The following video on Hebrew (in Hebrew but with English subtitles) is from Paul of the LangFocus youtube channel.