Anecdotally, what I've seen is that folks who learned typing in the 80s and earlier use two dashes '--' instead of the em-dash (although modern word processors seem to replace this combination with the em-dash). Something else I've noticed is their tendency to use two blank spaces between sentences.

I'm a self-taught typist, with all the quirks that comes with (can type programming stuff very accurately at a 100+ WPM; can type normal stuff at a high WPM as well, but the error rate goes up).