You've gotten some really good advice and Wyandottes7 gave a good general list.
I would only add that it seems there is always one of those in the crowd that doesn't fit into either category for some time and you simply have to wait it out until they either crow or lay an egg. (Which is what keeps this forum busy with posts.) So don't be in too big a hurry to declare "foul" (pun intended).
Usually it is the early developing females whose comb gets bigger and reddish quickly, and have thick legs, and act snotty, that throw you off...at least for me. I've given away a hen as a roo simply because she matured early and I thought she was going to be a roo and wanted her out before she took her attitude to crowing. Turns out she laid eggs at 13 weeks (way early)! Who'd of thunk.
Slower developing males can leave you guessing for a long time too. That can happen if you have a dominant rooster already and the juvenile roo stays quietly in the background.
It also gets harder to tell when you have mixed breed stock or poor hatchery quality stock as they won't develop according to standards and can be confusing.
Overall, I find my most reliable first indicator has been the size of comb combined with comb color (keeping in mind the breed type)....large and red (not just rosy-ish, dark pinkish). Then I watch and wait for the male feathers to show up.
It can help to take a flash light out and look at your flock as they roost after dark. Roo-ish feathers start coming in around 10 weeks and will have a high gloss shine on them. I found I can see them first with the flashlight before I can see them visually in the day.
Oh one last bit of advice...it is always easier to sex the OTHER guy's flock than your own.
Lady of McCamley