Usually you keep the unsexed fry in a tank where you can clearly see them and then you remove all the males as soon as they develop. When the anal fin is kept folded and starts to become concave, you have a male. If you don't remove him within a few days, he will impregnate every female in the tank.
Platys are usually done by about five months, if they still have an anal fin at 3/4 adult size, they are not going to become male. Swords on the other hand can look female for a very long time, over a year in some cases, and will only develop secondary sexual characteristics when not in the company of a dominant male. If kept in a tank with a fully developed adult male they will simply look like females but never get pregnant. I can usually spot them, but it takes experience. What you look for is thickening of the last two rays of the anal fin, the last four rays of the caudal fin, an upwards bend to the lateral line, and the shape of the stomach region and anus. It's not always easy and it's very easy to miss one and buy it as a female, or to invest a bit too much guesswork in hoping you end up with a male.
To add to the confusion, it is not heard of for very old females to develop a gonopodium and sword, and stop having fry, but they will never be fertile males!
In case it interests you, I have found one other reliable way of sexing platy fry much earlier, sometimes as young as a week or fortnight old. However, it does not work with all strains. In some strains, particularly wagtails, dark mickey mouse, tuxedo and some other dark colours, there is often a dark line on the back facing edge of the anal fin of the female. This usually develops very early. In some fish it is the first trace of colour to show up. I have never, ever seen a fish with this line develop a gonopodium. I'm not swearing it never happens, but I have raised many hundreds if not thousands of young platys and personally I have never seen one become male after developing this marking. I will get a photo of a fish displaying it if I can.