It really depends on what breeds you have, and what method you want to use for sexing. Vent sexing can be done at birth, but you would need training or a lot of experience to be able to properly vent sex. Some breeds are sex-linked, that is, males and females have different characteristics (feather color, pattern, etc.). Another method is feather sexing, but this also requires that the birds be a specific breed (fast feathering x slow feathering parents). With feather sexing, you look at the primaries (wing feathers) on DAY ONE after hatching; females have 2 rows of feathers, males have one.
An iffy method that I sometimes use to get a feel of how many cockerels or pullets I have is by observing their behavior. It's obviously not nearly as precise, but if you observe the chicks a lot and start to notice patterns, you can sometimes make educated guesses about who's going to turn out to be what gender. In general (and I really mean in general, this is not a precise method), males tend to hold themselves more vertically, neck stretched out, etc. Pullets hold themselves lower to the ground. These behaviors are most evident when the chicks are startled. If there is a sudden noise or movement, the chicks will all freeze for a few moments; the ones who habitually freeze in a more stretched out, upright position are more likely males, and the ones who huddle close to the ground are generally females.
Most people can start sexing chicks by physical appearance by about a month old (sometimes younger, sometimes older). The physical characteristics you should be looking for in pullets are: smaller, lighter-colored combs, faster feathering, straight tail feathers, no pointed hackle feathers, thinner, more delicate legs, and a longer, finer facial structure. In cockerels: thicker, stronger legs, slower feathering, faster-growing, larger, pinker combs, curved tail feathers, pointed hackle feathers, and a shorter, more "masculine"-looking facial structure.
However, different breeds are going to look and develop very differently (different feathering rates, different colored combs, different legs, build, etc.). It is easiest to determine male vs. female if you have more than one of a breed. Look within your chicks of that breed and see which ones are feathering out first (females), which ones have combs that are growing/pinking faster (males), which ones have stockier legs (males), etc.
What breeds do you have? We might be able to tell you if there are any obvious differences between the sexes in particular breeds. Good luck!