David Shaw wrote:
 > I am trying to write a scenario for either Call or Trail of Cthulhu
 > where the PCs are investigating a series of grizzly murders. My
 > questions are, with modern forensics and with 1920/30's forensics,
 > would it be possible to tell
 > a) if a body was beheaded pre- or post-mortem and

Yes; the question for the pathologist is "Did the circulation stop
before decapitation?"

If there is no sign of bleeding (bruising) into the injured tissue on
inspection and microscopic examination then decapitation
occurred after death. The autopsy needs to be performed before
decomposition hides this.

Examination of the wound can also determine the broad type of instrument
used or forces applied (cutting, stabbing, chopping, blunt force).

Note that it is possible to cause decapitation without direct force to
the neck (classically, motorcyclists hitting their fully-helmeted heads
at speed transmitting shear/torsional forces).

Modern forensics may be able to detect traces of the attacker (e.g.
fingerprints on the victim's skin, DNA residues.) CT scanning is another
very useful tool for forensics and archaeology after the mid 1970s.

 > b) if someone bleeds to death, how quickly they did so?

Yes.

This is inferred from the location of the holes in the circulation
and the likely flow through those holes. The matching volume of blood
should be able to be accounted for within the victim or from their
history (where the bleeding started, mechanism of injury/disease, etc.)

Near decapitation should produce fatal exsanguination in about thirty
seconds from division of both carotid arteries and the jugular veins in
the absence of first aid attempts or protective vasospasm.

Some of the devices used routinely in operating theatres or intensive
care could remove or replace a blood volume in a few minutes.

The modern era will allow more precision in determining how long it took
to bleed out.

As previous posters have mentioned, the most important step is to decide
to proceed to autopsy in the first place.

For Call of Cthulhu, the autopsy may produce SAN loss because the
findings don't make sense e.g.
- the victim has been drained but the blood or its drainage point is
nowhere to be found;
- decapitation appears to have been caused by something grabbing the
victim and twisting;
- the circulation is filled with... a substance that is definitely not
blood;
- there appears to be some healing responses in fractured bones
continuing long after the circulation stopped

Rob O'Connor
Medico, PGY 23.
I have had many discussions with forensic pathologists in my time.