Speaking solely from personal experience, by no means am I any kind of expert, but the baby chicks I've had that were the most aggressive during that first week do, in fact, turn out to be roosters. Sure, almost all chicks play fight, but it's usually very fleeting and done in passing. My chicks that turned out to be roosters, by the time they were six or seven weeks old, were having semi-formal little cock fights, and it was purposeful and lasted longer than the other kind of play fight.
In my current batch raised this summer, when they were six days old, I noticed one little guy who was rambunctious and aggressive. I thought he might be male, and I had ordered all pullets. When I examined him, his thicker legs and larger feet were a dead give-away. I took him back and exchanged him for another chick that they assured me was a pullet. They apologized and said he had gotten in with the pullets by mistake when some kids came into the store and handled all the chicks and put them down in the wrong pens.
When I got Penny home and put her in with the others, first thing she did was run around giving everyone a peck on the head, sort of like she was tasting them. As they grew, Penny was lead chick in everything. This last week, I noticed Penny was getting much larger than the rest of her mates. Her legs were thick and long, and her feet huge. She began sporting green tail feathers. I posted her photo here and everyone assured me Penny was certainly a rooster. Her name is now Penrod. I still call him Penny.
Again, this is my own opinion and experience. Mileage may vary.
