I started using a bread thermometer. Experimented with various breads and converged on trying to make a great basic french boule. Doing this at 6000 feet in Colorado in a conventional oven is challenging.
I could write at least 4000 words about bread, so I'll cut to the chase. I eventually got rid of the bread thermometer because I had my recipe down. The trick is measuring quantities (by weight, not volume) and using the same recipe, oven configuration and oven temperature every time. Once you figure out what works, that is. Once you eliminate variables and all you're left with is temperature or 'doneness' it's easy to just use time instead of temperature.
So to answer your question, I think they used to use trial and error, then consistency. Or perhaps a baker who taught the apprentice the exact method, perfected over time.
With a crisp loaf, you can just knock the underside and you'll get a reasonable idea of whether it's done. But a few fails (or not perfect results) will eventually get you to where you need to be.
Just for fun, here's some more detail: If you're interested in bread in your conventional oven, I'd highly recommend the awesome experience of making bread out of just flour, salt, yeast and water - a classic french recipe. And then hand knead it. Don't fall to the temptation of adding an egg yolk, sugar or olive oil just yet. It's like adding cocaine to soda. Of COURSE it will be more popular. But get the basics right first.
A few tips for your conventional oven: Get the thickest pizza stone you can get and a cast iron pan. Put the stone on a middle rack and the cast iron pan on the lower rack.
Use a moister dough (70 to 80% IIRC) and calculate the percentages of water vs flour based on weight. A scale is essential and a huge time saver. Make a nice wet dough and learn how to knead it. This will give you that wonderful crumb with big spaces. Learn about when to knead and when to rest. For better results, make an autolyse where you just lightly knead only the flour and water first and let it rest for 30 minutes before adding the yeast and salt and kneading. Sounds odd, but it gives amazing results. Calvel's technique (Julia Child's guru).
Preheat the hell out of the stone in the oven AND the cast iron pan under it. Boil a kettle full of water. Put thick gloves on for this next part.
When you put the bread into the oven, put the moist dough directly on the hot stone. Then immediately pour just a few ounces of the hot kettle water into the pan under the stone and shut the oven as soon as you can (with a face full of steam). You now probably realize that filling the kettle all the way saved you from having to tip the whole kettle into the oven and getting a nice steam burn. And those gloves were handy weren't they?
What you've just done is simulate a commercial baking oven in your crappy kitchen oven. The bread will rise suddenly and then the crust will start forming after about 7 mins. Let it get nice and crisp. Check it at around 30 to 45 mins depending on your oven temp. Use a bread thermometer. Take it out when it's 195. Flip upside down on a bread rack and let cool. Resist, resist, resist the urge to bust it open because you'll damage the fragile loaf at this point. After about 10 mins you can cut it if you want to serve hot bread.
When I was in my bread phase, my wife and I would eat hot bread fresh out of the oven with mature cheddar that would soften on the hot slices with red wine late at night.
The base receipe I used was
240 g flour,
0.35 l water,
5 g yeast,
And the most important part, 12 hour leavening time, no kneading and a cast iron pan preheated in the oven. Makes for very easy baking.