[Lagernoggin]

What physical evidence do you as GMs make available on a body killed by astral combat? Is the damage entirely mental (like matrix combat)? or does a blow from a spirit to the arm of an astral body, develop a wound on the arm of the physical body?

I know astral attacks can be stun or physical. I know dual-natured entities which suffered physical damage in astral combat would show physical damage according to their astral wounds. I know that if an astral body dies, the physical body continues in a coma if given life support.

As a GM, a character goes astral to defeat a spirit, and takes damage from the spirit that exceeds its physical condition monitor in astral combat, does not stabilize (leading to the death of the astral body), does not receive medical attention, and coma leads to death of the physical body. The PCs find this body, less that 24 hrs after the fact.
How do they discern what transpired from purely physical evidence? Astral evidence, should leave a signature of the spirit I would think, but if they don't or can't look, what should the physical remains reveal?

[Stahlseele]

That he got beaten up something fierce and electrocuted or bitten or whatever . .
There is no way to discern the difference, really . .  Only the complete missing of signs of combat could be a tip . .

[Valashar]

And although it's not physical evidence, everything astral leaves its signature behind for several hours.

Regarding forensic analysis of magic, I seem to remember some kind of fluff bit that had a detective discerning the type of spell that had damaged an area (was an acid spell with its area reduced in relation to its force IIRC). It would be more difficult, involving the ability to read what isn't there (like Stahlseele says). Deductive reasoning would be an asset.

[Charybdis]

Purely Astral combat (eg Astral Spirit and Projecting mage) by default has no effect or sign on the Physical plane, or the Mage's body.

You would need someone to Astrally assense the body and area to have any chance of determining what happened (and this would be dependent on the amount of time between incident and discovery, as well as power levels involved in the fight.
- Bigger powers = more astral disturbance in the area
- Longer time = less disturbance visible as the astral washes over the evidence like footprints on a beach

Physical spell effects and powers (Indirect Elemental spells, Powerballs, Flame Wall etc for example) will leave physical traces that can be forensically examined later (indefinitely in some cases) but these are immensely variable depending on the spell and power levels involved and environmental factors
Note: Physical spells will also leave Astral traces as well, which can be examined if found soon enough (before washing away

In the OP example, 24 hours is too long to get any information from the Astral space in the area surrounding the body (or the body itself).
Physically, what the PC's will find is a complete LACK of physical evidence (and the post-mortem will advise that the body was in perfect health before death... no poison, heart-attack etc).
This LACK of physical evidence will imply that an Astral attack of some sort is what killed the body (but there will be no forensic way to prove it.
Note: This is one reason why all mages are (supposedly legally registered, along with spells etc. It's almost impossible to track the use of spell powers etc after the fact, but if a registered mage is found dead in his house with no signs of physical trauma, it's easy for the Law to assume that a spirit (or other magical entity like a projecting mage) was involved, and then direct any investigation down a magical path.