Aggressive playing (chest bumping, jumping), "challenging" other chicks, and a regular upright stance when something gets its attention (photo above) are all possible early rooster signs. And while you'll see these behaviors randomly in most all chicks, it's the ones who continuously do them that are you best chances at being roosters.
However since very dominate females can also show these early signs regularly, I find it is best to also watch for some of the more subtle clues like physical changes. Some resources mention measuring and comparing wing feather length when the chicks are 2 days old, with females growing longer flight feathers first, but as we've found from experience, your eggs can hatch for several days leaving you with a batch of chicks that could include 4-5 days of age difference (which is A LOT when you're dealing with something that grows so fast).
Above are the same two chicks on the same sides. There is a big difference in the lower halves of their legs when held side by side. I hatched these chicks and they have the same set of parents, so I know I have a fair comparison here and not two different gene pools. But still, in general, you can usually start picking out thicker legs in the first month.
Sexing chicks can't be done on just one trait so make sure you observe and calculate the total picture. When you think you are seeing rooster traits in a chick, go ahead and make note of it. Possibly even tag the chick (we leg band them) so you can track and add up the traits over a period of time. With careful observation and making notes, you have a good chance of figuring out a good portion of who's who in your baby flock before they even leave the brooder.