Not with a microscope, but back before there was commercial feminized seed widely available, we used to sex out in veg, by pulling lanky tall (male) plants, and leaving stocky lush growing plants (typically female traits).
I grew out some bag-seed last year, and chose 100% females (4 out of 10), using this method.
At about 4 weeks right?
This happened to me recently (outdoor grow) and I was ecstatic to find out my plant was a female! But this has only happened once. Is it just more noticeable when the plant is a female? (This was my first). All the males have only shown me balls once they've been on 12/12 for about a week.