A thermometer is a "scaled" and therefore very accurate device in measuring body temperature, and thereby detecting fever.
What happened before thermometers were introduced in their current form in the 17th century? For instance, what was the usefulness and limitations of thermoscopes? And what might have preceded them other than the "feel" test?
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.
