How do we know Old English? Documentary sources are actually rather limited. They include:
The reason we have so many sources is that English was an isolated and Germanic language.
Therefore there is a parallel tradition of Latin & Old English literacy in the early Middle ages--700s-1000s.
Hence we have a lot of vernacular material in English that is earlier than that in Romance languages.
For Middle English, by contrast, we have lots and lots of written sources even though the language was not the official language of either government (French) or the church (Latin).