They didn't. The concept of food or beverages that taste the same, no matter when or where they're made is an extremely modern invention. It really only came about in the late 19th and early 20th century when industrial processes, statistical techniques, and physical sensors became well developed enough to reasonably guarantee the same outcome over and over again, for run after run. Prior to that, people just accepted a greater level of inconsistency in their foods, because that's all they'd ever known.
If you were a professional baker, or you had to bake all your own bread, I'd think you would become intimately familiar with the performance of your oven, with the characteristics of your leavening agent (probably a continually refreshed yeast or sourdough culture that you'd keep going for years), and with the local flour.
Although, since I'm currently reading through the Aubrey-Maturin novels, I wonder if the predominance of all these puddings they are always eating had something to do with them being easier to prepare reliably, compared to baked deserts.
I used to live round the corner from a baker that was still doing most steps by hand. I could tell from the shape of the rolls whether he was up long the day before. Practice makes you pretty consistent, but there are so many factors influencing the result that it's never exactly the same - which is part of the appeal.