If you are in veg stage long enough all plants will show preflowers but u dont have to wait for them to show preflowers u can force flower them wich is turning the lights to 12/12 when u want to and 41 days into veg is a normal time to start flowering mine are usually in veg stage for 2 months but some ppl onlt have them in veg for 3 or 4 weeks or until there is 4-7 nodes but yes all plants will show signs of sex if u keep them in veg stage long enough but if u force flower them just give em awhile n they will show signs soon.