If you want to be 100% sure you really have to wait until they're adults although with a bit of experience a keeper might find he/she can get a good idea at sub adult although even then they can still surprise you..I waited ages for a sub adult female to moult and when she did she moulted out to a penultimate male.
After they have moulted once and left the mothers back they will be called 2nd instar...R. junceus typically becomes adults at, male 6th instar, female 7th instar.
Edit:...Forgot to mention that from sub adult (5i male, 6i female) you should be able to make out the chelae dimorphism. Males have a bulbous growth inside and to the back of their chela that doesn't allow them to completly close their chelae..Females have the same growth but they also have an identation above the bulbous growth that allows them to close their chelae completly.
You should be able to find pics of this chelae dimorphism online or maybe even here....and you don't just have to search for R. junceus. As far as i know, all Rhopalurus spp have this same dimorphism.
