I have recently finished reading Cathy Reichs book Bones to Ashes. Part of the story involves the comparison of poems written by a girl aged about twelve and a grown up women. Both are French Canadian with English as their second language.
Though this book is a work of fiction I think the research would have been very thorough and the information about the use of words and phrases very interesting. So yes it is possible to tell the difference between whether it is a man or a woman typing is clearly visible to someone who is an expert on the subject.
As someone who finds UK accents fascinating as they are so often very localised it is interesting to watch the use of words relating to certain regions. Though I am not from the north east I find it great that people there can almost pinpoint where others come from with the difference in accents in Middlesborough, Newcastle and Sunderland areas.
As to chat and forums, everyone has their own individual style, some are very obvious others not so, but watching long enough it is often easy to recognise name changers or sex changers. These days when people have used chat for many years they become attuned to how others write so those who pretend they are something else are often easily spotted.