The easiest way is that if they are adults; the females tend to be wider and generally larger. There can be other markings but it depends a bit on the species.
It won't be easy to sex them until they are mature; at which point the female will be substantially larger. Males will be around 2 inches and females around 2 1/2 inches and substantially wider.
6 to 12 months depending on sex (from birth date). They are at least 3 months old when purchased - possibly substantially older. What size were they when you purchased them ?