Usually you can tell from just the look of the plant in my opinion. A male looks masculine, they are bigger, the nodes stretched farther apart, it grows faster. Meaning, don't get to excited if some plants are growing really well and getting big, it could be a male. On the other hand, don't always knock the runt of the bunch because that could be signs of a female. They look very feminine and petit, they grow more bushy. Just let time tell you the sex of your plant. Usually males will show signs well before females do so there is not much to worry about if your are watching your plants. Always start with 3 or more plants to increase the chance of getting a female and learn how to clone!