There will be a few people doing "but how do we know" and the answer is (as I understand it) with some uncertainty, structural clues in things like rhymes, where the written form clearly indicates sounds which had to sound alike. If you follow the clues through language shifts over the centuries, you can arrive at a sense which ones preserved the rhyme, and which didn't and a huge amount of inference added in gets you back where you expect things started.

The same with the great vowel shift.

Bill and Ted and So Crates.