I should say, “apparent” crime scene. Suppose there’s blood all over and it’s actually pig’s blood. How readily is it detected, and would investigators say, 50 years ago have had any trouble differentiating between human blood and pig’s blood?
I don’t need the answer fast 
The tests today would be able to easily tell the difference between human and pigs blood. 50 years ago? They would have only been able to know that it was blood. Hopefully, the samples were stored properly and tested again now. Crimes have been solved due to properly stored samples and modern testing.
Per http://en.wikipedia.org/wiki/Paul_Uhlenhuth:
Per the Forensic Medicine Archives Project University of Glasgow, Uhlenhuth did it in 1901 (or about 63 years before The Beatles visited the US.). FMAP goes on to mention some cases from 80+ years ago where human blood was distinguished from mammalian blood. For example from 1926:
The tests have been around since 1900 or so, and are easy enough. Though I imagine certain police departments still would have trouble with them…
The easiest way is the precipitin test To do this human blood is injected into a bunny. Then, after a few weeks the bunny’s serum will react instantly with all human blood, dried or fresh. Even in trace amounts. This was first used to prove murder in 1903, and and some point such bunny serum with preservatives became commercially available.
If the investigators had no reagent, and did not want to mess around catching rabbits and waiting, a simple ABO test would work. As long as the blood at the scene is still liquidy. All one would need for the test is a small amount of blood from a type A cop and some from a type B cop. (Punch them in the nose and hold a glass under it) And some way to spin the samples to separate the blood cells from the serum.
First the front type. Which would entail mixing the victim blood cells with cop serum. If the victim was a type A human, the cells would form into a clump when mixed with the B cops serum. If the victim was a type B human the blood would clump when mixed with the A cops serum. If no clumping then the victim is either a type O, or…not human.
Then the back type, which would entail mixing the victims serum with the cops blood cells. The serum from a type O human would clump both the A cops cells and the B cops cells. So if again no clumping…then victim a pig or chicken. Easy, but if before WWII then it is likely nobody around would know their blood type, and additional testing would be needed figure all that out.
Hope that answers it. Take care.
Sweet jumping jebus, that module in bio class pissed me off … I am ABneg, and the teacher didn’t believe me and they kept testing my blood convinced my lab partner and I screwed up. I knew for a damned fact I was ABneg, I had already been hospitalized once and since my elder [deceased] sister and father were both ABneg, I could make the wild assumption that I was probably ABneg even if I hadn’t had surgery with 3 units of blood to top me off … [Dad was a frequent offender\\donation to Red Cross because of the blood type thing. I grew up with him going off to donate every couple months regular as clockwork.]
You are the 1%. If the cops ever find your blood at a crime scene, they will get very very happy about it.
But, from a transfusion standpoint, your blood cells are worthless. They are rare, but will kill just about anyone else they are put into. Your plasma though…pure gold.