Yes preflowers usually start to show within 4-6 weeks of veg but tend not to fully develope until flowering. With experience you can usually differentiate the males and females not long (week or so) after the preflowers start to develope while in veg.
Yeah mine just at 8 weeks into veg and seen few grows and I am pretty sure i got females...can't wait to start flowering...One Question how long do you think you can veg a plant in 7 gallon pot for?