Sebrights are SO hard to tell, because the males are "hen-feathered" - so the comb is about the only way to tell the difference and it can take several weeks to begin to grow out into the male comb.
I couldn't tell at this stage. Wait for the comb to develop a spike pointing back along the top of its head. Then you know you have a cockerel.
Thanks for reading/commenting. I think I have three sebrights and one of them is making a errrr---eeeee noise. I can't tell which is making the noise but this one is the largest of the three and chest-bumps others often.
Sorry, that looks to be a boy. Sebright hens have VERY small, almost nonexistent, combs. The comb development on yours is a dead giveaway that it is a boy.