You can tell a great number of things about a person by the way they write, actually, if you know what to look for. You can tell what mood they're in, for example. People often make the mistake of assuming that because you can't read body language or hear tone through text, this isn't possible, but people write differently depending on their mood. The differences are subtle, but they are there. Many people have been shocked when I asked them what's wrong out of the blue, thinking they were hiding being sad, depressed, etc. Then they suddenly open up to me about whatever burden they've been carrying.
Yes, I can usually tell someone's gender by how they write. And I can't think that this has ever been affected by how far along they are in transition. Nor does it have anything to do with how 'cutesy' or verbose their writing is. I'm not sure I could explain it. It's just something I pick up on. Maybe it's part of my OCD powers, lol.