There are two main things to look for. Most importantly, the distance between the urinary and anal openings in female Gerbils is much smaller than it is in males. This method can be used reliably from at least three weeks of age, additionally, adult males have a large bulge at the base of the tail which is their scrotum. This is totally absent from females. This bulge is obvious by 6 weeks of age and can be seen at least two weeks earlier.
Most Gerbils become sexually mature by the age of three months old.
However if you pair a young female up with an older male you may find that she has her first litter earlier than if you had paired her up with a Gerbil the same age. Some pairs will breed earlier than others. Some pairs may take a year or more to produce a litter. Once she is sexually mature, a Gerbil will come into season every 4-6 days, usually in the evening. Mating will occur several times.