Well, bantam is a size, not a breed. It really depends on the breed of the bantam to tell gender by look. But when it comes to "sexing", they are to small to tell their internal structure.
Please post pictures in the category What Breed Or Gender is This? if you want a better response.
Do you know what breed it is? A picture will help us identify or go to the breed/genetics section and check out some of the chick pictures posted there. Feathersite also has a great baby chick section. http://www.feathersite.com/Poultry/BRKBabyChicks.html