Very informally.... Black dot above furrow is the first thing I look for. Then females have a more curved furrow and it's more prominent than the male's (of course of obvious the vent varies from sp to sp). Lastly, I look at the position of the anterior book lungs. Females book lunges usually curve away from the epigastric furrow giving a over all half moon appearance. In the male's the book lungs are usually more aligned with the furrow giving it an overall more straight appearance.
However some males have no obvious bullseye. sometimes a spider will have a more female looking vent area if it's been stuffed. Of course the only way to definitively sex a spider is through molt.