This is a fairly broad question, but basically it takes a good deal of scholarship. (And people who like to pick whatever snippets from Jewish sources that suits them, and post them online in all caps, aren't often so good at this.)
We do not have the power to overturn rabbinic law that was codified into the Talmud, however we can clarify the nature of those laws; sometimes a case that was an unusual loophole 1500 years ago is the one that we usually encounter today. (It reminds me very much of a first-aid book I'd read: "we used to recommend using meat tenderizer to treat certain animal stings, we no longer do. Medically it was sound advice, but the meat tenderizer you buy in the store today has different ingredients than the one sold a few decades ago." Or there was someone shouting "the Chofetz Chaim says it's prohibited to use an electric shaver!" Yes, but the electric shaver he prohibited was a device that employed a razor blade. That's not today's "electric shaver.") And some of what's in the Talmud isn't formalized rabbinic law, either.
When we get to something like Rambam's code, you have to dig into the commentaries to realize what Rambam is doing.
- Is he Doctor Rambam, giving medical advice? (If so, check if it fits within today's conventional medicine.)
- Is he Halachist Rambam, codifying a law in the Talmud? (If so it's binding, though check if other medieval authorities interpreted the Talmud that same way; and again make sure that whatever cases he was describing in the year 1200 are the ones we're describing today.)
- Or is he Preacher Rambam, giving practical applications of moral values, tailored to twelfth-century Egypt? The moral value is "modesty for women is good." Still applies. And you know what? "If you live in a massive estate in Alexandria in the year 1180 with extended family and you grow your own food within the estate, and the only reason you'd need to leave is to pay social calls, then maybe your social calls should be in moderation." Fine. But that doesn't address the applications that we normally encounter today.
(For that matter, there are instances 50 years ago where Rabbi Moshe Feinstein wrote "I think doing XYZ is listening to the yetzer hara", which was his social commentary -- "I think doing XYZ is heading in a bad direction." But his situations then may not be our situations today, either. For instance, "merging two small Orthodox synagogues is listening to the yetzer hara as people will expect a microphone in the big synagogue." And fifty years ago in America, they certainly did! Does that cover today's situations? Ask an Orthodox rabbi who is local to you in time, space, and sociology.)