How old is it?
You could try the upside down test... hold the chick up side down, if it goes limp = boy, if it trys to get right side up = girl. The asian chick shoppers use that on the straight run birds in the feed stores all the time to sex 'em. Though I have only seen this done with young chicks: So age may make a difference.
2 more methods I have read about but never tried: The second method: Picking the chick up by the scruff of the neck. If the chick lets it legs hang = a boy, if it draws its legs up to its breast = girl. A sexual response in the chick. The third method: Placing the chick on its back in your hand; females will stop kicking after a little while, and males will continue to kick.
I have never seen methods 2 or 3 and I suspect some methods work better and best at different ages. I have only seen #1 and used it after having the the ladies explain it to me I used the method. They had 100% on their testing and so did I after being schooled by them, but that was with younger chicks... when I looked on to the net people said their results varried.
Older Birds per Backyard Poultry Magazine:
In heavy breeds, cockerels will tend to feather in a patchy manner while pullets will feather more evenly. Cockerels also will begin to develop combs at an early age—males being distinguishable by three to four weeks in breeds with large single combs; at six weeks for males with a pea comb. In breeds in which the plumage color varies from male to female, males will begin to show their "true colors" at around six weeks of age. By eight weeks of age males of most breeds will begin to produce long, pointed saddle, hackle, and sickle feathers; females of all breeds tend to have broad, round feathers in these same sections. (This seems correct to me we had 1 roo once, and that was becuase we where given the chick and did not get to test it 1st).
There are differences in behavior we can notice as well. Cockerels tend to be generally bolder than pullets. They are less likely to scatter if you clap suddenly, whistle, or even wave a hat. When startled, cockerels will stand erect and give a warning chirp, while females are more likely to crouch down and remain silent (except Leghorns… so breed traits can effect responses). Female chicks may have wider pubic bones than male chicks, though this is only accurate depending on bloodline and selection for egg production. Males also tend to have larger feet and thicker legs than females.