[jfengel]

It's pretty much universal. Etymologists and lexicographers know that most words were in use for some time before being written -- anywhere from years to centuries. They try to make inferences by other means, as best they can.

They gradually expand the corpus they can search. A lot of words that are attributed to Shakespeare are gradually finding earlier sources, often in manuscripts. They knew all along that Shakespeare wasn't the first person to use a word (a common myth), but that his works were widely printed and thus survived.

Those manuscripts still don't include spoken usages, and show only the use by the class of people who could write. But it is solid data, before they go off into more tenuous hypotheses.

[ahazred8ta]

Some of the earliest recorded usages are found in court transcripts, where they wrote things down verbatim. One case from 1310 involved Roger F--kebythenavele. https://www.google.com/search?q=roger+by-the-navele