Alot of the time the ability to ID is due to a persons experience over time with certain breeds. Sometimes it is also a sex link characteristic. There really isn't a trick to it or a way to teach someone. You have to get your exprience in the trenches and know what to look for.
For example, speckedhen raises blue orpingtons and barred rocks. She knows those breeds well and with her trained eye she is able to give you identity keys to help determine the sex.
For the most part, unless it is a sex link characteristic you have to learn to read the body form and language to form an educated guess until such time it becomes obvious based on feathering, combs, wattles, behavior etc.
It would be nice if there was an easier way to do it.
I have one I know is a boy, and he is only 2 weeks old. He had a bout with pasty butt that lasted for days. Twice a day he had to sit on a warm wet pater towel, get his fanny cleaned and then get olive oil rubbed on his butt. When I told people at work what I had to do to treat pasty butt, all of the women said, "Yuck!" and all of the men said "Wow, olive oil wiped on the butt, that sounds great!" The chick actually seemed to enjoy getting cleaned and having oil smeared on. Therefore, I believe he will be a rooster. He actually seems to be upset when I hold him now and don't smear the oil on anymore.
"Wow, olive oil wiped on the butt, that sounds great!"
um, yuck LOL
My 4 year old daughter is smarter than I am! she told me "I know which ones are boys and which are girls!" I asked her how to tell-- "You watch 'em, if they lay a egg, it's a girl" LOL, she has more patience than I do!