Some strains/breeds are sex-linked, meaning that the boy chicks are different in color from the girl chicks. Other than that, there are two ways to tell, the Japanese sexing method and the wait-and-see method.
Wait and see = around four to five weeks of age, your chicks will have combs and wattles started. The larger the comb and wattle in a particular breed, the more likely that one is a roo. Roos also should be starting practicing their crows by then. This is really the only way to identify Silkies.
The Japanese method should only be done to new hatches and not at all to chicks older than one week of age. This involves holding the chick in your left hand a specific way (head held between pinkie and ring finger, legs held between index finger and middle finger, chick head down, then rubbing its belly with your right thumb until it poops. Wipe the poop away, then use your right index finger and thumb to apply pressure to the opposite sides of its vent. This will cause the vent to protrude a little. The boy parts -- a small pinhead-sized dark circle -- will be visible on the interior rim of the vent.
Again, if you have no experience doing this, please just wait. An inexperienced sexer can accidentally strangle a chick when doing this.