This is, of course, a difficult one, and there is more to this than most people realise. But here goes.
First, you have to notice the lady. So that is her looks. Her waist must be narrower than her hips and bust. She doesn't have to be young or slim nescessarily, but slim always helps. Neither does she have to be busty. But the curves have to show, otherwise she might as well be a guy.
Long hair helps to cue the eye. Then her face must have those feminine features, more delicate than a mans, the eyes and smile all have a role here.
When she walks, her gait has to have that feminine swing to it, from behind, her padding around the hips sways a particular way. The clothing should highlight all of this of course. The style itself, casual or formal actually doesn't matter. But the clothing should highlight the shape and motion. Dresses or skirts help to cue a guy as well.
Then once you notice the lady, then the personality kicks in. Being a little older now, funnily enough, it isn't all the bubbly stuff that I used to look for when I was younger. Now I look for her values. What does she choose to talk about. Is she into idle gossip, or is there a depth to her. Then after values, it is her wisdom. Does she actually apply her values, or are the values just a hypocritical front.
And finally does she have her psychological baggage under control. We all have baggage, but is it out front? Or is it managed?
Here is the clincher, after having said all that, in the last relationship I had, I got fooled into it by breast implants and blonde hair! I hung around too long and missed out on an opportunity with a much sexier lady. I suppose we are all human!