[Jeir]

Unfortunately with young long-haired syrians, it can be hard to tell. =D
Female: The openings should be very close together. Females also have nipple lines on their tummy. Also, smell. Female Syrians have a very distinct smell every 4 days, and may even freeze and 'present' to you if you stroke their backs during this time.
Males: The openings are further apart, and when they're a bit more mature, their testes can be very pronounced. They do not have nipple lines, and they don't have any real change in their smell.