Body temperature drops two degrees the first hour after death, and one-and-a-half degrees each successive hour. There are another five major ways to calculate time of death:
1. Livor mortis: body turns red/purple/blue within half an hour of death.
2. Rigor mortis: body begins to stiffen within 2 hours of death, is completely stiff within 6-12 hours of death, and begins to un-stiffen within another 36-48 hours (depending on climate).
3. Stomach contents: digestion rates are difficult to calculate, but we can determine at the very least what sort of food and hence which meal the victim last ate.
4. Amount of soft tissue that remains. Consult a forensic anthropologist for details.
5. Entomological factors: the presence of insects (and particularly blowflies) and their form (egg, larva, pupa, adult) can accurately calculate time of death, because different species of insect have specific generation times.
information source: www.cbs.com