I mean, we can’t know everything, but in the last couple of decades we’ve learned how to derive quite a bit about an animal from a fossil. Things like where muscles attach let us figure out the muscle structure, the size and structure of bones yields hints to what and where an organism’s weight was, and from the little nubs on the bones we can tell where feathers are. And all of that can come together and form a decent picture of what that animal’s behavior was. That’s just the stuff I’m aware of as a casual, I’m sure there’s tons more clues in fossils that I’m not aware of.