sexing plants

I only have experience starting plants for outside (18x6) this is first indoor only. With the 18on 6 off Icould sort out the males. Will I have to wait till i cut the light back to 12 and 12. I read somewhere it was said I could tell from clones. dosent make sense for me to clone then i.d. sex ! any advice . thankyou in advance.I am off to work.

generally, yes, but if you want to make it a slower transition (for more light) you could decrease the light schedule by an hour every few days until 12-12... some plants will show sex less than 12 hours dark.