Some breeds and strains are born with distinguishing characteristics from day one. It depends completely on which breeds/types you have.
In general, however, the roosters will have larger legs and knock knee look to their walk at 10 weeks. Almost from the beginning, they will be the ones who tend to look up and be alert, because that is their instinct and will be their job one day. They tend to hold themselves more erect, vertical spine. Finally, of course, they tend to get red combs and wattles while the female's remain yellowish and small until 15 weeks when their's too will turn red as they approach the beginning of lay.