So here's a link to the fascinating Gender genie. I tried it on several pieces of mine, and it was convinced the author was male in each case. Tried it on writings of other people, and it called gender correctly each time. Then, because I wanted to make things harder for it, I went and checked the writings of several transgendered people, and in each case it accurately called the gender of the writer as the one that the person had elected.
Apparently it only calls it correctly about 51% of the time, according to the "Am I Right?" stats, which may mean that I just got lucky...
Have fun playing with it.