Some strains will show sex @ week 4-8 of the vegging period.Yea,when you flip to the 12/12 light cycle is when you will know for sure the sex!
No signs....you have to change the light period to find out the sex!
If I were YOU...I would vegg for 4-6 weeks in the 3g Smart pot,then go to 12/12.Ive done it countless times in a 3g,with no rootbound plants.Now if you were to vegg for 8 weeks plus Id do the 5g.
If you dont have Fem seeds,and wanna know the sex take a cut and let it root usually 7-10 days then put the clone under 12/12 to find out the sex.Hate to see you vegg a male for 8 weeks.