Livor mortis- setting of blood in the lower portion of the body leaving a purplish red discoloration to the skin, takes 2-8hrs
Algor Mortis- the change in body temp following death.
Rigor Mortis- the stuffing of the skeletal muscles after death
Algor, Livor , and rigor Mortis can be used to estimate PMI by knowing how long ago the person died. By how stiff the corpse is, the cold body temperature, and either the discoloration of the skin.
Other changes to the body after death to determine the PMI would be to check out the stomach contents and changes to the eyes. For the stomach it takes 4-6 hours for the stomach to empty the contents to the small intestine. It then takes another 12 hours for the food to leave the small intestine. Approximately 24 hrs. from when a meal was eaten until all undigested food is released from the large intestine.
Another change in the body would be to the eyes. A thin film is observed within two-three hrs. if the eyes were open at death. If the eyes at death were closed then it would take 24 hours to be covered in the film.