Go by personality. You won't tell very well about people aggression until they start going through the hormone phase. This can start any where around 12 to 16 weeks of age and run through their first year.
All boys will try to attempt the boundaries; however, a people aggressive one will go beyond it to take you completely out as he wants the ultimate top bird slot.
I would recommend hanging on to the boys and watch how they develope. A real aggressive rooster will show his true colors as soon as he realizes the girls are fun. Generally, by 6 months you will already picked out the most aggressive and know who the good boy is.
A good boy is so worth finding. My Lester is a good boy that even the wild birds pay attention to his warnings. He escorts his ladies to the best spots, lets them have the yummy bugs he catches, always available to defend them, and is never rough during their whoopie time. He even does a cute dance before and after.
Lester is aloof and politely friendly toward us. The only time he pecked was when a hwn squeeled while I went to pick her up for inspection. Even then he was really shy about it.
Once you have your good boy and the girls all decide they approve of him you will find the new flock dynamics more peaceable. Ofcourse, the girls may fight over who gets to sleep next to him at roost time.