Agree with Gerald, 8 weeks is the minimum for determining sex with certainty. Some people have their pet methods but they're not proven since after all it's a 50/50 chance you're right.
It's a 50/50 chance is you're guessing blind. Perhaps the genetics of my flock make prediction easier to do early? More than just color go into my guesses, but that's the main one. Other things I look at are weight and behavior. Ask yourself this, have you ever seen a really young chick mount another chick and had that chick end up female? Ever had any really large female chicks (I'll look up the weights).
Two years ago I sold 5 or 6 to someone for cheap because I was pretty sure they were male. Would it surprise you to know that I just spoke with her and all are male?
The dark pied, BS, and two whites that I hatched from DylansMom I guessed correctly, I think, but should go back and verify that, lol.
The 12 from last year that I still have I sexed correctly *well* before 8 weeks.
Same with other years... I always have a few that are obviously female or male at a very early age.
-Kathy