1) Is it blood?
2) If it is blood, from what species does it originate? (Otherwise, is it human?)
3) If of human origins, how closely can it be tied to a particular individual?
To answer the first question, is it blood, evidence collectors can start with tests on the crime scene. There are three primary field tests, each with benefits and deficiencies.
1) Benzidine color test was an alcohol based oxidizer, meaning that it interacted with oxygen to identify blood from other specimens. It caused cancer, however, so was replaced.
2) Kastle-Meyer color test was the replacement. It used phenolphthalein and hydrogen peroxide mixed with the stain. If the stain was blood, the mixture would turn pink. This test was not very efficient and was messy.
3) Luminol uses a reagent or chemical that bonds with blood then reacts with black light. The blood will luminesce or glow under black light. The benefits of luminol include its cost, its ability to detect blood up to 10,000 times diluted, and its ease of use. The drawback is that it destroys blood factors, preventing further tests on the blood. Because of this, luminol found blood is photographer quickly and blood sites are collected before luminol is used to track the placement of blood.
Scientists can also use the Takayama and Teichmann test. It is a microcrystalline test done in the lab but it takes time, is less sensitive to blood, and is less efficient in its detection.
Other things to note are that dried blood is harder to analyze but blood aged 10-15 years has still given good results for court use and even 4-5 thousand year old mummies have given positive blood results. Blood that is 11 years old yielded blood types. Blood tests can also identify use of drugs like methadone and marijuana. In fact, marijuana has been identified to one-millionth of a gram in blood.