I can't help with sexing Brahma chicks, but this is my light Brahma rooster at four months. He is enormous, has longer tail feathers than anyone else and has long neck feathers. I couldn't tell any difference between him and the other chickens until a few weeks ago. We can't have roosters either and got a silkie rooster too in our batch of ten straight run chicks. If there's anyone out there in southern California, you're welcome to him!
Matter of fact, we are a bit stressed because despite all of our precautions (chicken designated boots, roof on the coop, chicken clothes, showers and cleaning shoes after a trip to the feed store), we seem to have Marek's. Sigh. Once chicken seems to be recovering, two are in a dog crate doing well but walking like drunken sailors, and one had one day of spasms and that was it.