It is possible to tell the sex of a parakeet from the band of flesh that grows across its nostrils, called a cere. A male parakeet will have a smooth bluish or purple cere, while the cere on a female will be tan or brown and will be rough in appearance. Only parakeets at or approaching physical maturity will be differentiated in this manner.
There is no known same sex aggression between bird species. If you do not wish for your birds to breed, then purchase two birds of the same sex to keep together. You may wish to purchase a small cage to move one of the birds to during mating season as an alternative to purchasing two or more of the same sex.