I think we can rule out the Lavender Orpington- she started laying small pullet eggs about a week and a half ago that I was able to confirm are hers, they're distinctly smaller than the ones we had in the incubator (some of which would not fit through the gap in the rotating tray on day 18 and had to be manually picked up and replaced).
Today the older ones are three weeks and a day old, out of the yellow ones this one really stands out as a potential rooster candidate:
The features that stand out are: bigger comb than most of the yellow siblings, bigger size than anyone else, noticibly thicker legs, and red wattles seem to be growing. (Not related to gender, but next to each eye there's a black dot, no other sibling has that feature.) By comparison, here's a chick I'd suspect of looking more like a pullet:
And maybe these, except for the BR on the right who keeps looking roostery (and with red in wattles).
That one by the feeder really doesn't appear to show those features.
I was picking several of them up without much commotion, as there are a few that keep running up to me - the BR rooster acts like a puppy, and there's a yellow one with red wattles that's not as big but is curious about everything. The largest one is very brave, but acts more like Arthur - reserved and walking around with authority. When I finally picked that one up, everyone else suddenly froze until I put him back down again.
Among the younger group which will be two weeks tomorrow, the two black ones do not yet have any tail feathers.