GHB is one of the so-called Date Rape Drugs—along with Ecstasy, Rohypnol, and Ketamine. I have an article on these on my website (See Below).
GHB has been difficult to detect, primarily because it’s rapidly metabolized (destroyed) by the body. But new techniques employing Nuclear Magnetic Resonance (NMR) Spectroscopy allow the detection of GHB metabolites (breakdown products) as much as 24 hours later. This gives investigators a longer time period to uncover GHB in a victim.
GHB can also often be found in the victim’s hair up to a month or more after exposure, but this testing is not as yet perfected.
More on the fascinating world of Forensic Toxicology can be found in FORENSICS FOR DUMMIES: