Hens lay eggs, roosters don’t. Less certain is that most roosters crow but it is really rare to see a hen crow. Basically I’m saying it’s not always easy.
There are very few auto sexing breeds but there are a few. What breed are they? There are a few crosses, not breeds, that can be sex linked at hatch, but the parents have to be set up a certain way. This only works for crosses. Do you know what color and breed the parents were? Some specialists can look inside the vent and tell but I’m not trained to vent sex.
For the vast majority of chicks you cannot tell for sure early on. There is just no real difference in appearance or behavior. Often at about five weeks some differences do start to show up. I’ll give a link that talks about that. Sometimes it becomes really obvious, even before five weeks, but a lot of times it takes a lot more than five weeks to be sure. A lot of the time these things are more clues that they might be male or female instead of absolute ID’s. It’s not always easy but you do get better with experience.
How to sex chicks