I think it's in the hips and pelvic bones, mainly. Reptiles and some dinosaurs have a sprawling, splayed-out posture. Other dinosaurs, birds, and mammals have

I think it's in the hips and pelvic bones, mainly. Reptiles and some dinosaurs have a sprawling, splayed-out posture. Other dinosaurs, birds, and mammals have independently-evolved upright postures. The birds apparently evolved from the splayed-hip dinosaurs rather than from the upright-hip dinosaurs, so a change in hip structure could be a decent dividing line. But strictly speaking, birds (Aves) have wings as their forelimbs, rather than arms. The (extinct) moa is an exception in that it had no wings, but that is because it had no forelimbs at all. It is very likely that other theropods besides the birds also had feathers, but it is rare for soft tissues to fossilize, so it can be hard to say what the scale and feather coverage was like for a given species when all you have is a partial skeleton. Determining coloration is almost impossible, though it has been estimated from especially well-preserved fossilized impressions of feathers that showed different spacing between the microstructures on the feather.