I know that I personally have never so far been able to tell men from women over a purely text channel without having been told explicitly

I have. There are text analyzers which give statistical likelihoods on the gender of the author of a given piece of writing. They generally give fairly wide confidence margins, but their algorithms are pretty simple and they don't apply a lot of heuristics that humans can use. Even the best gender analyzer can only guess with limited confidence, but a person's writing style offers considerably more than zero information about their gender.