Just from my own quantitative observations, some degree of gynecomastia seems to be a decent indicator that your body is pre-programmed to both respond to estrogens, as well as store fat in the breast area.
It's not necessarily the be all end-all, because one of my friends with no chest whatsoever pre-hormones has developed a full B+ after 4 years of hormones, one of my internet trans friends grew into a D cup from almost nothing, and a LOT of girls do indeed develop breasts from nothing like this. But there's also a lot of them who never got past an A, whereas most of the trans women I know who had some degree of "man boobs" ended up at least in the B range, so the consistency seems to be a bit higher.