Before the invention of thermometer, there was no notion of "temperature",
as a physical quantity, a number. (To define a physical quantity one needs to specify a way to measure it). Only qualitative differences could be discussed:
"hot", "cold", "fever" etc.
Fever is easily detected without any thermometer. By physical contact of the person who detects it (presumably having normal temperature). Even now, when thermometer is not available fever is easily detected by touching the person's forehead with one's hand or better with the lips.
Mercury thermometers measured reliably with 0.1 degree Celsius accuracy, but this
accuracy is rarely required. Since the use of mercury was prohibited, electronic thermometers are used which is much less reliable and less accurate. But one does not really need to measure temperature with such high precision.