Guppies start to show signs of their sex after a few weeks. Yor best indicator will be the presence/absence of the gravid spot.
As for your male/female ratio, you want to aim for 3-4 females for each male. Since the males are constantly trying to mate with the females, this will allow them to spread the harassment to other females, allowing the mated female to recover.
It should also be known that female guppies can store the males sperm for 3+ months. Is is not uncommon for you to find pregnant guppies long after any males have been removed from the tank.