FACT: your pupils dilate when you see the person you are attracted to.
Because the nervous system controls the muscles of the irises, the response of the nervous system to different stimuli results in involuntary pupil dilation. Another commonly cited reason the pupils dilate is in response to excitement or sexual arousal. When a person sees something or someone he finds very attractive, his eyes may dilate.
Also, try this: if you are attracted to some woman and you work or spend time with her, you can tell when she's ovulating if you just follow your body's reactions. That is, if you get a raging stiffy and she looks even hotter than usually, bingo. Your nose tells you what's going in her body.
We truly are just a dressed up monkeys :)