Well, some people just have experience with that breed and they know things about certain breeds, like I have experience with Silkies, Production Red, and new hampshire.
So, here's the basic signs. The tail feathers will have a greenish sheen, and the wattles and comb will be bigger and redder. With certain breeds, like silkies, if the crest is slicked back rather than poofy then its probably male, some are sexlinked, the males will also have bigger legs, and with crested, like polish the crest will be wilder and bigger. xD that was a mouthful