Usually after the first week pistils emerge for females but it can take up to 2 weeks. Males will show in the first couple of days if you are paying close attention.
Females tend to look more like new growth if you see pistil before you're flowering...then it will look like a crystal-like hair (this is for collecting pollen) The week you see any start of sexing you'll be able to tell the sex...I'm still on my first grow but this has proven so for all 7 of my plants 6 females 1 male!