If you look at Bratabase, they have a measure of how wide the gore (center piece between boobs) is, which is sort of a proxy for that. Also gore height, so you know how high up it goes. But basically the only underwire option for us is plunges, and sometimes those are also made with excessive interboob distance in mind so you get to have your boobs fall out the middle...

(...yes indeed I did have bras and bra fitting as a special interest for a bit.)