If the language is truly dead, they can’t. It is mostly guesswork.
They can get a rough idea sometimes by finding another language that isn’t dead and finding old writings comparing them. They also will sometimes find that different modern languages share words from the dead language and pronounce them the same, then work from those to guess at the pronunciation of others.
Also, with a language like Latin, scholars and the Catholic Church kept using it long after the Roman Empire and they both kept records of scholars discussing the changing pronunciations. This not only helps determine the correct pronunciation, but the pronunciation of words from other languages sometimes was compared to Latin, allowing Latin to tell us how other languages were pronounced.
But in reality, it is almost all just guesses. They do not know.
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