Depends on the breed. Some breeds can be color-sexed. Others can be feather sexed. Lots can only be 'vent' sexed - by a trained professional.
MOST breeds you can start seeing gender differences pretty clearly by the time they are fully feathered out (around 6 wks). The boys will usually have much darker and larger combs and wattles than the girls.
Another couple-three weeks and the differences are more and more apparent.
What breeds/age chicks are you talking about?? You can always post pics of chicks in the 'what gender/breed' category and see what the group thinks.
My silkies were pretty ambigious until just recently and they were hatched out mid-November. It was at least 14 weeks or so until I was absolutely sure of their gender (and then they just may be fooling me...) We were sure that Mr. White was a boy (much larger than the others, etc) until all the sudden we realized there were others developing streamers and ear patches and combs and she wasn't. Now she's Mrs. White, much to my hubby's glee since he REALLY likes that chicken and we knew if she were a 'he' he'd have to go!
In silkies the signs to look for are 'streamers' coming off the back of the head pouf in boys. Boys also tend to have brighter combs and ear patches earlier than the girls.
Still with silkies some can fool ya until you hear them crow or see they lay an egg.
Good luck with your chicks. Silkies are a lot of fun and so cute.