Is there a way to tell if a breast is composed of more breast tissue or fat tissue?

I was wondering because in a sports bra, a woman can have her breasts completely flattened by the bra and appear flat chested. Others look almost the same in a sports bra. That could be down to differences in the sports bras but I tested this and noticed that I can't really compress my breasts that flat. Also, when women lose weight some maintain their breast size and some lose a lot of volume there. Does this mean that some people have breasts composed more of breast tissue than fat tissue? And is there a way to feel the difference between the two tissues or would that only be visible on some kind of scan?