The best and in my opinion the only way that you can sex a plant without the worrie of any plants getting pollonated, is to clone then root them then put on 12/12 off.you could still do this by topping the plants but if they have been veging for the amount of time you say then i would put on 12/12 off then keep a close eye on them.But for future grows,always flower clones this is the only way like i said more than not unless you have been growing for a good few years.If you are new to growing then there is too much risk,by this i mean you could miss one male that shows its sex late and this will cause all kinds of problems.I would say that there is a few things you can keep a eye out for though the larger plants are often more likely to be males,also follow the stem up where a set of brances nodes come away from the stem look for any small tear dropped shapes,any always go with if in dought throw it out.