When can I tell Sex

Four weeks in from seeds and now have 7 nodes. I can't tell if they are males or females and want to seperate without pollination. I know where to look but do not see any hairs. I just transplanted and see they have adjusted well and would also like to know how much to adjust there watering amount. it used to be about 10 oz. When I transplanted each plant took a whole gallon. The hydro guy says to flush with nutes? Seems like a waste of money. Is it necessary to flush. I am using coco to grow in. I was going to flower at 18 inches. they are at 9 now and very bushy with a thick trunk. I am using all canna nutes with t-5 lights and temp controlled. The Humidity has alot of fluctuation and will use dehumidifier when I go to flower Any advice would be appreciated.

thanks,
Hobbit

gotta put it in 12/12 before you can tell.
then wait bout 2 or so weeks and look for sacs starting or hairs.
hairs= good
sacs=bad