I can tell by scent. Scent of the man, not of his jizz. His scent will tell me how compatible we will be. Men who smell a certain way always turn out to be dicks. Men who smell another way tend to be players, which may or may not fit my agenda. The men I wanted to keep to myself and make a life with have all smelled similarly to each other. I'm definitely a woman who could point out my man blindfolded and hands bound, just from the scent of his damp, sweaty shirt.
But semen? I can't tell anything from semen. Unless you count the time I told this one dude he was diabetic because his jizz consistently tasted like candy. I drank it like he was a vending machine. Good gawd I always had my mouth on his dick! But seriously. Spooge is not supposed to be sweet like that, and sure enough he was outrageously, off-the-scale diabetic. I may have saved his life.