Final answer:
PSA (Prostate Specific Antigen) is the protein indicating the presence of semen in a sample, with acid phosphatase test and immunoassays as detection techniques.
Step-by-step explanation:
The presence of prostate specific antigen (PSA) indicates that a sample stain contains semen. PSA is a protein enzyme also known as a serine protease that is produced by the prostate and is normally present in semen where it acts as an anticoagulant to keep the semen liquid. The two techniques commonly used to detect PSA in forensic analysis are the acid phosphatase test, where a purple color change indicates the presence of the enzyme, and the use of immunoassays, such as enzyme-linked immunosorbent assay (ELISA), which can specifically identify and quantify PSA.