My daughter has been hatching chickens for science this school year so we have been doing some strange but fun things including candling daily and trying out most of the "sexing" methods old wives tales and more.
Feather sexing I have found to be quite accurate with the heavy breeds for the first 3 days..I have found between 24-48 hours I have been seldom wrong.It's not accurate though for your lighter or mixed breeds where you don't know what's there...but still fun to do
The pullets do tend to feather out faster
Combs tend to grow faster on the little roo's but can be misleading with the larger combed pullets until your used to seeing them
One I found interesting and does have some merit...take a hat or something like it overhead of the chicks and wave it around to startle them some...roos will stretch up towards it pullets will squat.Amazingly this has worked to a much higher percentage than I would have thought.
The easiest way is to wait
Some breeds you can tell within a couple of weeks...others are much harder to tell(darn those peacombs) I can pick my little leghorn roosters out after just one week
Still though the hatcheries only guarantee you 90% on all pullets because it's not that easy to tell and you can be wrong a lot