The graivd spot appears as a darkend triangle just behind the stomach and above the anal fin. Swordtails give birth every 30-50 days depending on a lot of factors. When they are getting close to birthing they will start looking more squared off. The gravid spot will also be quite dark and become also squarish rather then a triangle. Swordtails have no mating rituals I have ever heard of. The females show no interest and the males go after them constantly. The act takes about 1 second and the female can birth 4-6 times from one insemination. Do not put the female in a small trap or net too early or she will stress and can abort. Keep a close eye on them and you will learn after a time how long it takes. Keep a journal and then you will know how long a particular fish goes between births. You are likely to lose a lot of the fry in the first few batches. Relax there will be more soon on the way. Maybe even too many HTH