We know because of the saddle feathers. They are thin and hanging low along the body, a hen would just have regular feathers here. That and the fact that the tail is completely green, not just a few feathers in the middle.
You can see his saddle feathers really well in the second picture you posted, where his butt is toward the camera. Saddle feathers are the ones in front of his tail along his back. Now look at the barred rock hen behind him in the photo. See how her feathers are just regular rounded feathers there?
you do not have to have chicks ...... if you do not want chicks.... just keep gathering your eggs a few times a day and you will never have chicks. Roosters also protect the hens
Thanks for your excellent explanation. The saddle feathers and green tail are a dead give-away! We just didnt' know and some of the books are a little vauge. So far Bridget (now named Frankie by the kids) has caused zero problems; - not that the only man in a group of seven women would ha ha. We will just keep them the way we have for now. If he ever becomes a meanie we'll just address the problems then. Thanks to all who post on such a great site!!!! Here's a pic of Darby with her favorite girl Ming-Ming: