The more round the inside of the eyebrows and pointing closer to the forehead, the more feminine they appear. On the other hand, the more the eyebrows are pressed towards the eyes, the more masculine the eyebrows appear. Furthermore, the eyes are more flat and horizontal for men and the eyebrows are far more arched and the density decreases as the tail of the eyebrows arch downwards for women.
See more on the Feminine Secrets site.
I have an appointment this Friday for an eyebrow consultation. Growing up I had a complete mono-brow so shaping and cleaning the center has been a face routine since teen. The center is no longer a problem, but the old age bushiness and lack of color (grey) is. Being mostly gray they tend to be almost invisible so I have used in the past an eyebrow pencil to make them show up. I will have the "brow artist take before and after. This is a little scary, but not as big a step as growing out my hair.