Here's an example of a primitive beer being brewed:
Http://www.garshol.priv.no/blog/394.html
"When we arrived, they had gotten the fire going, and were heating the brew liquor. Outside, in the yard, they'd set up the mashtun, and were getting ready to start mashing.
Vytautas poured a sack of pale malts from Viking Malts in Panevežys into the tun, then added buckets of boiling hot water while his son Ignas stirred the mash. They were using water, not juniper infusion, because this was Lithuania, not Norway. Vytautas kept adding water, and eventually started lifting the mash paddle, staring intently at the drops running off it. Then he would add some more water, then repeat. Finally, he was satisfied. Now they could pack up the mashtun and let it stand.
But I'd seen no thermometer, so how could he know this was the right mash temperature? I went over to check with my own thermometer. It showed 65C. Simonas saw my surprise and explained to me what was going on. Since Vytautas was using boiling water the temperature of the water was always the same. So if he could hit the right ratio of water to malts, the temperature of the mix would also be roughly right. And this is what Vytautas was doing when he was staring at the paddle: he was judging the ratio of water to malts."
As mentioned, step mashing can also be done without a thermometer with the use of decoction or infusions. A step mash helps increase extraction and fermentability from under-modified malt.
Edit: If historical farmhouse brewing interests you, you can pre-order Lars Garshol's upcoming book:
https://www.amazon.com/Historical-Brewing-Techniques-Lost-Farmhouse/dp/1938469550/