How is it found out how long someone been dead?
If it's only been a few hours, a forensic pathologist can put a thermometer into the middle of the liver. The liver is a big organ which cools down from body temperature at a predictable rate. Also things like rigor mortis and lividity (the blood pools at the bottom of a body so whichever way the body is situated, the lower parts turn red.) For longer periods, days, weeks, months, there are other indicators. Counting insect eggs or larvae (maggots) is one way. Also degree of 'putrefaction'.
Rate of decomposition in accordance to the environment. Where temp etc.
Watch CSI. Liver temperature is one indicator. The types of bugs living/hatching are another indicator. There are other indicators.
Medical experts can tell by things like body temperature and stage of decomposition.
Jake No Chat
There are several ways to approximate the time of death. Body temperature, the progress of rigor-mortis, and the broken watch/clock that seems to turn up at 90% of the crime scenes. At the time of death, a condition called "primary flaccidity" occurs. Following this, the muscles stiffen in rigor-mortis. All muscles in the body are affected. Starting between two and six hours following death, rigor mortis begins with the eyelids, neck, and jaw.
from the insects that are in them
The temperature of their body which gets cooler the longer the person is dead and the stiffness of the body and there are many other signs of decay that a pathologist discovers when they examine the body and the organs.
There are a few different ways that would be different for different environmental conditions. There's to much to explain in this type of forum
Sometimes it's impossible on others there is a trail of breadcrumbs to follow.