There are women that you enjoy spending time with and there are women that just do something to you and for me it's in the eyes and the balls.
Some women you can just look at and get an overwhelming desire to want, again and again and again.. Also for me the tell tale sign is the kiss, i don't get hard often through kissing but there have been 3 women in my life who just by a decent kiss ( nothing overly passionate ), would get me erect. It's those women that i find my male friends saying "easy tiger" as they know i used to fall hard for!