A common indicator that something has been slipped into someone's drink is that the person becomes impaired beyond what she has consumed and then wakes up at home, at someone else's house, or during sex. Drugs are now ideal for sexual assaults. The problem is that many of these drugs pass through the system so quickly that there is usually no positive toxicology, meaning that the urine tests don't show anything. Another confounding factor is that people don't tell all the drugs they've used recently, not wanting to admit drug use, so they complicate the toxicology analysis.
People also do a lot of drugs and alcohol consensually together.
She said that some of the sexual assault victims show up at the emergency rooms still intoxicated and so they are treated as drug overdoses and evidence is lost. More educational work needs to be done with emergency professionals to make them aware of the problems. A big GHB rape case in LA recently which ended up with multiple life sentences where some DJ was knocking both women and men out at parties and having sex with them. She said that if a suspect has a recipe on their computer for GHB, that helps their prosecution case. They described a game where people were all given glasses to drink from and then sometime during the cocktail party, someone went around and put stickers on peoples glasses to show them how easy it would be to slip something in their drink undetected.