Not sure about the brown tips, but you won't be able to sex your plants until you change your light cycle to 12/12. Once the cycle is changed, the plants will usually show their sex anywhere from 3-14 days.

If you don't want to put the whole plant into fruiting, you can just cover one branch with some heavy opaque cloth or paper for 6 of the 18 hour light cycle so that branch thinks it is on a 12/12 cycle -- you just have to remember to be consistent and make sure you are there for the covering/uncovering.