you can pelvic sex them... the cockerels pelvic bones will be more pointed and almost touching... pullets at 4 months will be almost far enough apart to fit a finger between the bones... it's worked for me so far. Hold the bird facing you with its body in the palm of your hand and slide your middle or ring finger to its bum... you'll feel the bones.
What about the method of holding them briefly upside down holding their legs....if they lay back right away-they are hens and if they pull up and fight, they are roosters??? Any validity to that method?
I had a guy come out and look at some birds I had listed on craigslist. I told him they were hens but I guess he didn't believe me. He picked a couple up and layed them on their backs... the birds raised all kinds of racket and tried like heck to get away from him. He determined that they were both roosters because of that. I then pointed out to him that they both had bare bellies from where they were recently broody and then showed him the eggs they had layed that day... eggs were still in their pen and they were the only 2 birds in there. He was insistent that they were not hens... like the others have said, it doesn't work... or maybe it does and I have egg laying roosters?!?
silkieowner - 3 months is still a little young... by 3 1/2-4 I can usually tell if I have several birds of the same age to compare... 4+ mos are much easier to tell.