Suspected semen stains, usually found with the use of an alternate light source, can be screened at the crime scene by use of the BCIP (5-bromo-4-chloro-3-indolyl phosphate) reagent.  In this presumptive test, the BCIP reagent reacts with prostatic acid phosphatase, a constituent of semen, to give a blue or aqua color reaction.  A blue color change is a positive reaction and is indicative of the presence of semen.

The image below shows a positive BCIP test on the left and a negative test on the right.