The crest sexing isn't reliable at first, because sometimes they can fool you! The best way to tell is around 4-6 weeks. Roosters will start to get red wattles and/or a V comb (which looks like little horns) and pullets definitely will not. The only thing a pullet at 4-6 weeks should have is possibly tiny flesh colored wattles, and many don't even have that. Some roosters will only get wattles or only a comb at first, then the second thing will show up later, but as long as they have one or the other then it's 99.5% sure it's a roo!
I've found this to be reliable for crevecoeurs, polish, spitzhaubens, and houdans! I think sultans are similar also because the only sultan I've had grew a comb at 6 weeks and he was a rooster.